UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED
MANAGEMENT INVESTMENT COMPANIES

ITEM 1.                                                  REPORT TO STOCKHOLDERS.

The Semi-Annual Report to Stockholders is filed herewith.

Semi-Annual Report May 31, 2013

WESTERN ASSET MUNICIPAL DEFINED OPPORTUNITY TRUST INC.
(MTT)

Fund objectives

The Fund’s primary investment objective is to provide high current income exempt from federal income tax* and then to liquidate on or about April 30, 2021 and distribute all of the Fund’s net assets to shareholders. As a secondary investment objective, the Fund will seek total return. There can be no assurance the Fund’s investment objectives will be achieved.

*  Certain investors may be subject to the federal alternative minimum tax (“AMT”), and state and local taxes will apply. Capital gains, if any, are fully taxable. Please consult your personal tax or legal adviser.

What’s inside

Dear Shareholder,

We are pleased to provide the semi-annual report of Western Asset Municipal Defined Opportunity Trust Inc. for the six-month reporting period ended May 31, 2013. Please read on for Fund performance information and a detailed look at prevailing economic and market conditions during the Fund’s reporting period.

I am pleased to introduce myself as the new Chairman, President and Chief Executive Officer of the Fund, succeeding R. Jay Gerken, as he embarks upon his retirement. Jay has most recently served as Chairman, President and Chief Executive Officer of the Fund and other funds in the Legg Mason complex. On behalf of all our shareholders and the Fund’s Board of Directors, I would like to thank Jay for his vision and guidance, and wish him all the best.

I am honored to have been appointed to my new role with the Fund. During my 23 year career in the financial industry, I have seen it evolve and expand. Despite these changes, keeping an unwavering focus on our shareholders and their needs remains paramount. This was a consistent focus of Jay’s, and I look forward to following his lead in the years to come.

As always, we remain committed to providing you with excellent service and a full spectrum of investment choices. We also remain committed to supplementing the support you receive from your financial advisor. One way we accomplish this is through our website, www.lmcef.com. Here you can gain immediate access to market and investment information, including:

·  Fund prices and performance,

·  Market insights and commentaries from our portfolio managers, and

·  A host of educational resources.

We look forward to helping you meet your financial goals.

Sincerely,

Kenneth D. Fuller

Chairman, President and Chief Executive Officer

June 28, 2013

Investment commentary

Economic review

The U.S. economy continued to grow over the six months ended May 31, 2013 (the “reporting period”), but the pace was far from robust. Looking back, U.S. gross domestic product (“GDP”)i growth, as reported by the U.S. Department of Commerce, was 1.3% in the second quarter of 2012. Economic growth accelerated to 3.1% in the third quarter, partially due to increased private inventory investment, higher federal government spending and moderating imports. However, economic activity sharply moderated in the fourth quarter, with GDP expanding an anemic 0.4%. This was driven by a reversal of the above factors, as private inventory investment and federal government spending weakened. Economic growth then improved, as the U.S. Department of Commerce’s final reading for first quarter 2013 GDP growth, released after the reporting period ended, was 1.8%. Accelerating growth was due, in part, to strengthening consumer spending, which rose 2.6% during the first quarter, versus a 1.8% increase during the previous quarter.

While there was some improvement in the U.S. job market, unemployment remained elevated throughout the reporting period. When the period began, unemployment, as reported by the U.S. Department of Labor, was 7.8%. The unemployment rate fluctuated between 7.8% and 7.9% through January 2013. Unemployment then fell to 7.7% in February, 7.6% in March and 7.5% in April, before edging up to 7.6% in May. In an encouraging sign, the number of longer-term unemployed has declined in recent months. In February 2013, more than 40% of the people without a job had been out of work for more than six months. This fell to 37.3% in May 2013.

Meanwhile, the housing market brightened, as sales generally improved and home prices continued to rebound. According to the National Association of Realtors (“NAR”), existing-home sales rose 4.2% on a seasonally adjusted basis in May 2013 versus the previous month and were 12.9% higher than in May 2012. In addition, the NAR reported that the median existing-home price for all housing types was $208,000 in May 2013, up 15.4% from May 2012. This marked the fifteenth consecutive month that home prices rose compared to the same period a year earlier. While the inventory of homes available for sale rose 3.3% in May 2013 to a 5.1 month supply at the current sales pace, it was 10.1% lower than in May 2012.

While manufacturing activity was weak in many international developed countries, it was generally positive in the U.S. Based on the Institute for Supply Management’s Purchasing Managers’ Index (“PMI”)ii, the U.S. manufacturing sector expanded during the first five months of the reporting period. However, manufacturing then experienced a setback, falling from 50.7 in April 2013 to 49.0 in May (a reading below 50 indicates a contraction, whereas a reading above 50 indicates an expansion). During May, 10 of the 18 industries within the PMI expanded, versus 14 expanding the prior month.

Investment commentary (cont’d)

Market review

Q. How did the Federal Reserve Board (“Fed”)iii respond to the economic environment?

A. The Fed took a number of actions as it sought to meet its dual mandate of fostering maximum employment and price stability. As has been the case since December 2008, the Fed kept the federal funds rateiv at a historically low range between zero and 0.25%. At its September 2012 meeting, prior to the beginning of the reporting period, the Fed announced a third round of quantitative easing (“QE3”), which involves purchasing $40 billion each month of agency mortgage-backed securities (“MBS”) on an open-end basis. In addition, the Fed further extended the duration that it expects to keep the federal funds rate on hold, until at least mid-2015. At its meeting in December, the Fed announced that it would continue purchasing $40 billion per month of agency MBS, as well as initially purchasing $45 billion a month of longer-term Treasuries. The Fed also said that it would keep the federal funds rate on hold “...as long as the unemployment rate remains above 6.5%, inflation between one and two years ahead is projected to be no more than a half percentage point above the Committee’s 2.0% longer-run goal, and longer-term inflation expectations continue to be well anchored.” At its meeting that ended on June 19, 2013, after the reporting period ended, the Fed did not make any material changes to its official policy statement. However, in a press conference following the meeting, Fed Chairman Bernanke said “...the Committee currently anticipates that it would be appropriate to moderate the monthly pace of purchases later this year; and if the subsequent data remain broadly aligned with our current expectations for the economy, we would continue to reduce the pace of purchases in measured steps through the first half of next year, ending purchases around midyear.”

Q. Did Treasury yields trend higher or lower during the six months ended May 31, 2013?

A. Both short- and long-term Treasury yields moved higher during the reporting period. When the period began, the yield on the two-year Treasury was 0.25%. It fell as low as 0.20% on April 29, 2013 and was as high as 0.30% on several occasions, including at the end of the period. The yield on the ten-year Treasury began the period at 1.62%. Ten-year Treasuries hit a low of 1.59% in December 2012 and peaked at 2.16% at the end of the period.

Q. What factors impacted the spread sectors (non-Treasuries) during the reporting period?

A. The spread sectors experienced periods of volatility during the period given a number of macro issues, including the European sovereign debt crisis, mixed economic data and concerns related to the U.S. “fiscal cliff” and sequestration. However, the majority of spread sectors modestly outperformed equal-durationv Treasuries given generally solid demand from investors looking to generate incremental yield in the low interest rate environment.

Q. How did the municipal bond market perform versus the taxable bond market over the reporting period?

A. The municipal bond market performed largely in line with its taxable bond counterpart during the six months ended May 31, 2013. Over that period, the Barclays Municipal Bond Indexvi and the Barclays U.S. Aggregate Indexvii fell 1.09% and 1.05%, respectively. The municipal bond market was weak in December 2012, due to concerns about the future tax-favored status for municipal securities. The municipal bond market then rallied over three of the next four months as investor demand was generally solid, tax revenues improved and default rates remained low. However, the municipal bond market again weakened in May, along with the overall fixed-income market, as interest rates moved sharply higher.

Performance review

For the six months ended May 31, 2013, Western Asset Municipal Defined Opportunity Trust Inc. returned -0.16% based on its net asset value (“NAV”)viii and -9.44% based on its New York Stock Exchange (“NYSE”) market price per share. The Fund’s unmanaged benchmark, the Barclays Municipal Bond Index, returned -1.09% for the same period. The Lipper General and Insured Municipal Debt (Unleveraged) Closed-End Funds

Category Averageix returned -0.24% over the same time frame. Please note that Lipper performance returns are based on each fund’s NAV.

Certain investors may be subject to the federal alternative minimum tax, and state and local taxes will apply. Capital gains, if any, are fully taxable. Please consult your personal tax or legal adviser.

During this six-month period, the Fund made distributions to shareholders totaling $0.50 per share, which may have included a return of capital. The performance table shows the Fund’s six-month total return based on its NAV and market price as of May 31, 2013. Past performance is no guarantee of future results.

Performance Snapshot as of May 31, 2013 (unaudited)

All figures represent past performance and are not a guarantee of future results. Performance figures for periods shorter than one year represent cumulative figures and are not annualized.

Looking for additional information?

The Fund is traded under the symbol “MTT” and its closing market price is available in most newspapers under the NYSE listings. The daily NAV is available on-line under the symbol “XMTTX” on most financial websites. Barron’s and the Wall Street Journal’s Monday edition both carry closed-end fund tables that provide additional information. In addition, the Fund issues a quarterly press release that can be found on most major financial websites as well as www.lmcef.com.

In a continuing effort to provide information concerning the Fund, shareholders may call 1-888-777-0102 (toll free), Monday through Friday from 8:00 a.m. to 5:30 p.m. Eastern Time, for the Fund’s current NAV, market price and other information.

As always, thank you for your confidence in our stewardship of your assets.

Sincerely,

Kenneth D. Fuller

Chairman, President and Chief Executive Officer

June 28, 2013

RISKS: The Fund’s investments are subject to credit risk, inflation risk and interest rate risk. As interest rates rise, bond prices fall, reducing the value of the fixed-income securities held by the Fund. The Fund may invest in lower rated high-yield bonds which are subject to greater credit risk (risk of default) than higher-rated obligations. Municipal securities purchased by the Fund may be adversely affected by changes in the financial condition of municipal issuers and insurers, regulatory and political developments, uncertainties and public perceptions, and other factors. The Fund may use derivatives, such as options and futures, which can be illiquid, may disproportionately increase losses, and have a potentially large impact on Fund performance. The Fund may invest up to 10% of its assets in securities that have the economic effects of leverage which can increase the risk and volatility of the Fund. Shares of closed-end exchange-traded funds may trade at a discount or premium to their original offering price and often trade at a discount to their NAV.

All investments are subject to risk including the possible loss of principal. Past performance is no guarantee of future results. All index performance reflects no

Investment commentary (cont’d)

deduction for fees, expenses or taxes. Please note that an investor cannot invest directly in an index.

The information provided is not intended to be a forecast of future events, a guarantee of future results or investment advice. Views expressed may differ from those of the firm as a whole.

Fund at a glance† (unaudited)

Investment breakdown (%) as a percent of total investments

†      The bar graph above represents the composition of the Fund’s investments as of May 31, 2013 and November 30, 2012 and does not include derivatives, such as futures contracts. The Fund is actively managed. As a result, the composition of the Fund’s investments is subject to change at anytime.

Spread duration (unaudited)

Economic Exposure — May 31, 2013

Spread duration measures the sensitivity to changes in spreads. The spread over Treasuries is the annual risk-premium demanded by investors to hold non-Treasury securities. Spread duration is quantified as the % change in price resulting from a 100 basis points change in spreads. For a security with positive spread duration, an increase in spreads would result in a price decline and a decline in spreads would result in a price increase. This chart highlights the market sector exposure of the Fund’s portfolio and the exposure relative to the selected benchmark as of the end of the reporting period.

Effective duration (unaudited)

Interest Rate Exposure — May 31, 2013

Effective duration measures the sensitivity to changes in relevant interest rates. Effective duration is quantified as the % change in price resulting from a 100 basis points change in interest rates. For a security with positive effective duration, an increase in interest rates would result in a price decline and a decline in interest rates would result in a price increase. This chart highlights the interest rate exposure of the Fund’s sectors relative to the selected benchmark sectors as of the end of the reporting period.

Schedule of investments (unaudited)

May 31, 2013

Western Asset Municipal Defined Opportunity Trust Inc.

See Notes to Financial Statements.

Western Asset Municipal Defined Opportunity Trust Inc.

See Notes to Financial Statements.

Schedule of investments (unaudited) (cont’d)

May 31, 2013

Western Asset Municipal Defined Opportunity Trust Inc.

See Notes to Financial Statements.

Western Asset Municipal Defined Opportunity Trust Inc.

Abbreviations used in this schedule:

Summary of Investments by Industry†

†  As a percentage of total investments. Please note that Fund holdings are as of May 31, 2013 and are subject to change.

Ratings Table*

Standard & Poor’s/Moody’s/Fitch**

See Notes to Financial Statements.

Statement of assets and liabilities (unaudited)

May 31, 2013

See Notes to Financial Statements.

Statement of operations (unaudited)

For the Six Months Ended May 31, 2013

See Notes to Financial Statements.

Statements of changes in net assets

See Notes to Financial Statements.

Financial highlights

For a share of capital stock outstanding throughout each year ended November 30, unless otherwise noted:

1              For the six months ended May 31, 2013 (unaudited).

2              Per share amounts have been calculated using the average shares method.

3              For the period March 27, 2009 (commencement of operations) to November 30, 2009.

4              Initial public offering price of $20.00 per share less offering costs and sales load totaling $0.94 per share.

5              Performance figures may reflect compensating balance arrangements, fee waivers and/or expense reimbursements. In the absence of compensating balance arrangements, fee waivers and/or expense reimbursements, the total return would have been lower. Past performance is no guarantee of future results. Total returns for periods of less than one year are not annualized.

6              The total return calculation assumes that distributions are reinvested at NAV. Prior to January 1, 2012, the total return calculation assumed the reinvestment of all distributions in accordance with the Fund’s dividend reinvestment plan. Past performance is no guarantee of future results. Total returns for periods of less than one year are not annualized.

7       The total return calculation assumes that distributions are reinvested in accordance with the Fund’s dividend reinvestment plan. Past performance is no guarantee of future results. Total returns for periods of less than one year are not annualized.

8       Annualized.

9              The impact of compensating balance arrangements, if any, was less than 0.01%.

10         The investment manager has agreed to reimburse all organization expenses.

11         Amount represents less than 1%.

See Notes to Financial Statements.

Notes to financial statements (unaudited)

1. Organization and significant accounting policies

Western Asset Municipal Defined Opportunity Trust Inc. (the “Fund”) was incorporated in Maryland on January 15, 2009 and is registered as a non-diversified, closed-end management investment company under the Investment Company Act of 1940, as amended (the “1940 Act”). The Fund’s primary investment objective is to provide high current income exempt from federal income tax and then to liquidate on or about April 30, 2021 and distribute all of the Fund’s net assets to shareholders. As a secondary investment objective, the Fund will seek total return. There can be no assurance the Fund’s investment objectives will be achieved.

The following are significant accounting policies consistently followed by the Fund and are in conformity with U.S. generally accepted accounting principles (“GAAP”). Estimates and assumptions are required to be made regarding assets, liabilities and changes in net assets resulting from operations when financial statements are prepared. Changes in the economic environment, financial markets and any other parameters used in determining these estimates could cause actual results to differ. Subsequent events have been evaluated through the date the financial statements were issued.

(a) Investment valuation. The valuations for fixed income securities (which may include, but are not limited to, corporate, government, municipal, mortgage-backed, collateralized mortgage obligations and asset-backed securities) and certain derivative instruments are typically the prices supplied by independent third party pricing services, which may use market prices or broker/dealer quotations or a variety of valuation techniques and methodologies. The independent third party pricing services use inputs that are observable such as issuer details, interest rates, yield curves, prepayment speeds, credit risks/spreads, default rates and quoted prices for similar securities. Short-term fixed income securities that will mature in 60 days or less are valued at amortized cost, unless it is determined that using this method would not reflect an investment’s fair value. Futures contracts are valued daily at the settlement price established by the board of trade or exchange on which they are traded. If independent third party pricing services are unable to supply prices for a portfolio investment, or if the prices supplied are deemed by the manager to be unreliable, the market price may be determined by the manager using quotations from one or more broker/dealers or at the transaction price if the security has recently been purchased and no value has yet been obtained from a pricing service or pricing broker. When reliable prices are not readily available, such as when the value of a security has been significantly affected by events after the close of the exchange or market on which the security is principally traded, but before the Fund calculates its net asset value, the Fund values these securities as determined in accordance with procedures approved by the Fund’s Board of Directors.

The Board of Directors is responsible for the valuation process and has delegated the supervision of the daily valuation process to the Legg Mason North American Fund Valuation Committee (the “Valuation Committee”). The Valuation Committee, pursuant to the policies adopted by the Board of Directors, is responsible for making fair value determinations, evaluating the effectiveness of the Fund’s pricing policies, and reporting to the Board of Directors. When determining the reliability of third party pricing information for investments owned by the Fund, the Valuation Committee, among other things, conducts due diligence reviews of pricing vendors, monitors the daily change in prices and reviews transactions among market participants.

The Valuation Committee will consider pricing methodologies it deems relevant and appropriate when making fair value determinations. Examples of possible methodologies include, but are not limited to, multiple of earnings; discount from market of a similar freely traded security; discounted

cash-flow analysis; book value or a multiple thereof; risk premium/yield analysis; yield to maturity; and/or fundamental investment analysis. The Valuation Committee will also consider factors it deems relevant and appropriate in light of the facts and circumstances. Examples of possible factors include, but are not limited to, the type of security; the issuer’s financial statements; the purchase price of the security; the discount from market value of unrestricted securities of the same class at the time of purchase; analysts’ research and observations from financial institutions; information regarding any transactions or offers with respect to the security; the existence of merger proposals or tender offers affecting the security; the price and extent of public trading in similar securities of the issuer or comparable companies; and the existence of a shelf registration for restricted securities.

For each portfolio security that has been fair valued pursuant to the policies adopted by the Board of Directors, the fair value price is compared against the last available and next available market quotations. The Valuation Committee reviews the results of such back testing monthly and fair valuation occurrences are reported to the Board of Directors quarterly.

The Fund uses valuation techniques to measure fair value that are consistent with the market approach and/or income approach, depending on the type of security and the particular circumstance. The market approach uses prices and other relevant information generated by market transactions involving identical or comparable securities. The income approach uses valuation techniques to discount estimated future cash flows to present value.

GAAP establishes a disclosure hierarchy that categorizes the inputs to valuation techniques used to value assets and liabilities at measurement date. These inputs are summarized in the three broad levels listed below:

·             Level 1 — quoted prices in active markets for identical investments

·             Level 2 — other significant observable inputs (including quoted prices for similar investments, interest rates, prepayment speeds, credit risk, etc.)

·             Level 3 — significant unobservable inputs (including the Fund’s own assumptions in determining the fair value of investments)

The inputs or methodologies used to value securities are not necessarily an indication of the risk associated with investing in those securities.

The following is a summary of the inputs used in valuing the Fund’s assets carried at fair value: