IGI Western Asset Investment Grade Defined Opportunity Trust Inc

Fund objectives

The Fund’s primary investment objective is to provide current income and then to liquidate and distribute substantially all of the Fund’s net assets to stockholders on or about December 2, 2024. As a secondary investment objective, the Fund will seek capital appreciation. There can be no assurance the Fund will achieve its investment objectives.

The Fund seeks to achieve its investment objectives by investing, under normal market conditions, at least 80% of its net assets in investment grade corporate fixed income securities of varying maturities.

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Letter from the chairman

Dear Shareholder,

We are pleased to provide the annual report of Western Asset Investment Grade Defined Opportunity Trust Inc. for the twelve-month reporting period ended November 30, 2020. Please read on for a detailed look at prevailing economic and market conditions during the Fund’s reporting period and to learn how those conditions have affected Fund performance.

Special shareholder notice

On July 31, 2020, Franklin Resources, Inc. (“Franklin Resources”) acquired Legg Mason, Inc. (“Legg Mason”) in an all-cash transaction. As a result of the transaction, Legg Mason Partners Fund Advisor, LLC (“LMPFA”) and the subadvisers became indirect, wholly-owned subsidiaries of Franklin Resources. Under the Investment Company Act of 1940, as amended, consummation of the transaction automatically terminated the management and subadvisory agreements that were in place for the Fund prior to the transaction. The Fund’s manager and subadvisers continue to provide uninterrupted services with respect to the Fund pursuant to new management and subadvisory agreements that were approved by Fund shareholders.

Franklin Resources, whose principal executive offices are at One Franklin Parkway, San Mateo, California 94403, is a global investment management organization operating, together with its subsidiaries, as Franklin Templeton. As of November 30, 2020, after giving effect to the transaction described above, Franklin Templeton’s asset management operations had aggregate assets under management of approximately $1.5 trillion.

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:

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Letter from the chairman

We look forward to helping you meet your financial goals.

Sincerely,

Jane Trust, CFA

Chairman, President and Chief Executive Officer

December 31, 2020

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Fund overview

Q. What is the Fund’s investment strategy?

A. The Fund’s primary investment objective is to provide current income and then to liquidate and distribute substantially all of the Fund’s net assets to stockholders on or about December 2, 2024. As a secondary investment objective, the Fund will seek capital appreciation. There can be no assurance the Fund will achieve its investment objectives.

The Fund seeks to achieve its investment objectives by investing, under normal market conditions, at least 80% of its net assets in investment grade corporate fixed income securities of varying maturities. The Fund may invest up to 20% of its net assets in corporate fixed income securities of below investment grade quality (commonly known as “high yield” or “junk” bonds) at the time of investment and other securities, including obligations of the U.S. government, its agencies or instrumentalities, common stocks, warrants and depositary receipts. While the Fund may invest up to 20% of its net assets in below investment grade securities, the Fund will, under normal market conditions, maintain a portfolio with an overall dollar-weighted average of investment grade credit quality. The Fund may invest up to 20% of its net assets in securities of foreign issuers located anywhere in the world, including issuers located in emerging market countries. Additionally, the Fund may invest up to 20% of its net assets in non-U.S. dollar denominated securities.

The Fund may invest in derivative instruments, such as options contracts, futures contracts, options on futures contracts, indexed securities, credit default swaps and other swap agreements, provided that the Fund’s exposure to derivative instruments, as measured by the total notional amount of all such instruments, will not exceed 20% of its net assets.

In purchasing securities and other investments for the Fund, we may take full advantage of the entire range of maturities and durationsⁱ offered by corporate fixed income securities and may adjust the average maturity or duration of the Fund’s portfolio from time to time, depending on our assessment of the relative yields available on securities of different maturities and durations and our expectations of future changes in interest rates.

The Fund may take on leveraging risk by utilizing certain management techniques, whereby it will segregate liquid assets, enter into offsetting transactions or own positions covering its obligations. To the extent the Fund covers its commitment under such a portfolio management technique, such instrument will not be considered a senior security for the purposes of the Investment Company Act of 1940. However, as a fundamental policy, the Fund will not leverage its capital structure by issuing senior securities such as preferred shares or debt instruments.

At Western Asset Management Company, LLC (“Western Asset”), the Fund’s subadviser, we utilize a fixed income team approach, with decisions derived from interaction among various investment management sector specialists. The sector teams are comprised of Western Asset’s senior portfolio management personnel, research analysts and an in-house economist. Under this team approach, management of client fixed income portfolios will reflect a consensus of interdisciplinary views within the Western Asset organization. The

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Fund overview (cont’d)

individuals responsible for development of investment strategy, day-to-day portfolio management, oversight and coordination of the Fund are S. Kenneth Leech, Michael C. Buchanan and Ryan K. Brist.

Q. What were the overall market conditions during the Fund’s reporting period?

A. Fixed income markets, in general, posted mixed results over the twelve-month reporting period ended November 30, 2020. Most spread sectors (non-Treasuries) lagged equal duration Treasuries amid periods of heightened volatility. This was driven by a number of factors, including extreme risk aversion as the COVID-19 pandemic escalated, sharply falling global growth, aggressive monetary policy accommodation from the Federal Reserve Board (“the Fed”)ⁱⁱ, ongoing trade conflicts and a number of geopolitical issues.

Both short- and long-term U.S. Treasury yields moved sharply lower during the reporting period. The yield for the two-year Treasury note began the reporting period at 1.61% and rose as high as 1.66% on December 12, 2019. The low for the period of 0.11% occurred several times toward the end of July 2020, the beginning of August 2020, and the end of September 2020, and ended the period at 0.16%. The yield for the ten-year Treasury began the reporting period at 1.78% and moved as high as 1.93% on December 23, 2019. The low of 0.52% occurred on August 4, 2020 and ended the period at 0.84%.

All told, the Bloomberg Barclays U.S. Aggregate Indexⁱⁱⁱ returned 7.28% for the twelve months ended November 30, 2020. Comparatively, the Bloomberg Barclays U.S. Credit Index^iv returned 9.17 over the same period and the Bloomberg Barclays U.S. High Yield — 2% Issuer Cap Index^v returned 7.17%.

Q. How did we respond to these changing market conditions?

A. We reduced the Fund’s allocation to investment grade corporate bonds. In terms of sectors, we increased the Fund’s allocations to Industrials and Energy, while paring its allocations to Consumer Staples, Materials and Financials. Elsewhere, we pared the Fund’s emerging market exposure. From a credit quality perspective, we increased the Fund’s allocation to securities rated BB and reduced the overweight to securities rated BBB. Finally, we adjusted the Fund’s yield curve^vi positioning. In particular, we reduced the Fund’s exposure to the long end of the yield curve.

During the reporting period, U.S. Treasury futures were used to manage the Fund’s duration and yield curve positioning. They detracted from performance. Corporate investment grade index swaps, which were used for credit hedging purposes, modestly contributed to results.

Performance review

For the twelve months ended November 30, 2020, Western Asset Investment Grade Defined Opportunity Trust Inc. returned 8.96% based on its net asset value (“NAV”)^vii and 5.06% based on its New York Stock Exchange (“NYSE”) market price per share. The Fund’s unmanaged benchmark, the Bloomberg Barclays U.S. Credit Index, returned 9.17% for the same period. The Lipper Corporate Debt BBB-Rated Closed-End Funds Category Average^viii returned 8.74% over the same time frame. Please note that Lipper performance returns are based on each fund’s NAV.

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During the twelve-month period, the Fund made distributions to shareholders totaling $0.85 per share.* The performance table shows the Fund’s twelve-month total return based on its NAV and market price as of November 30, 2020. Past performance is no guarantee of future results.

All figures represent past performance and are not a guarantee of future results.

** Total returns are based on changes in NAV or market price, respectively. Returns reflect the deduction of all Fund expenses, including management fees, operating expenses, and other Fund expenses. Returns do not reflect the deduction of brokerage commissions or taxes that investors may pay on distributions or the sale of shares.

† Total return assumes the reinvestment of all distributions at NAV.

‡ Total return assumes the reinvestment of all distributions in additional shares in accordance with the Fund’s Dividend Reinvestment Plan.

Q. What were the leading contributors to performance?

A. The largest contributor to the Fund’s relative performance during the reporting period was security selection. The Fund’s holdings in the banking industry were the most beneficial, with overweight positions in Credit Suisse and Barclays adding the most value. We have a favorable view on the banking space, as loan loss reserves protecting major banks’ capital bases are at all-time highs. Elsewhere, in the Information Technology sector, an underweight to Intel contributed to returns, as we were of the view that valuations were stretched coming into 2020. Finally, in the Consumer Non-Cyclical¹ sector, an overweight to CVS Health Corp. was beneficial due to management’s focus on debt reduction along with attractive relative value coming into the year.

From a sector allocation perspective, overweights to outperforming Financials (specifically banking and insurance) were positive for results.

Q. What were the leading detractors from performance?

A. The largest detractor from the Fund’s relative performance during the reporting period was its quality biases. In particular, a large overweight to securities rated BBB detracted from results, as they underperformed higher quality investment grade bonds.

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Fund overview (cont’d)

Duration and yield curve positioning were headwinds for returns. More specifically, a short duration and an underweight to the long end of the yield curve were not rewarded. In terms of sector allocation, an underweight to Information Technology was negative for performance.

Finally, overweight positions in Occidental Petroleum and Marathon Oil detracted from returns. Occidental Petroleum posted poor fundamental results and was downgraded to below investment grade status during the reporting period. Its bond prices did improve toward the end of the period, but all told, the overweight detracted from results. Additionally, the oil price war between Saudi Arabia and Russia earlier this year, along with the demand shock due to COVID-19, significantly affected the profitability of both issuers given the sharp decline in commodity prices.

Looking for additional information?

The Fund is traded under the symbol “IGI” and its closing market price is available in most newspapers under the NYSE listings. The daily NAV is available online under the symbol “XIGIX” 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 (click on the name of the Fund).

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.

Thank you for your investment in Western Asset Investment Grade Defined Opportunity Trust Inc. As always, we appreciate that you have chosen us to manage your assets and we remain focused on achieving the Fund’s investment goals.

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Sincerely,

Western Asset Management Company, LLC

December 15, 2020

RISKS: The Fund is a non-diversified, limited term, closed-end management investment company designed primarily as a long-term investment and not as a trading vehicle .The Fund is not intended to be a complete investment program and, due to the uncertainty inherent in all investments, there can be no assurance that the Fund will achieve its investment objectives. The Fund’s common stock is traded on the New York Stock Exchange. Similar to stocks, the Fund’s share price will fluctuate with market conditions and, at the time of sale, may be worth more or less than the original investment. Shares of closed-end funds often trade at a discount to their net asset value. Because the Fund is non-diversified, it may be more susceptible to economic, political or regulatory events than a diversified fund. The Fund’s investments are subject to a number of risks, including credit risk, inflation risk and interest rate risk. As interest rates rise, bond prices fall, reducing the value of the Fund’s holdings. The Fund may invest in lower-rated high-yield bonds or “junk bonds”, which are subject to greater liquidity and credit risk (risk of default) than higher-rated obligations. 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 in securities or engage in transactions that have the economic effects of leverage which can increase the risk and volatility of the Fund. The Fund may also invest in money market funds, including funds affiliated with the Fund’s manager and subadvisers. For more information on Fund risks, see Summary of information regarding the Fund - Principal Risk Factors in this report.

Portfolio holdings and breakdowns are as of November 30, 2020 and are subject to change and may not be representative of the portfolio managers’ current or future investments. Please refer to pages 8 through 26 for a list and percentage breakdown of the Fund’s holdings.

The mention of sector breakdowns is for informational purposes only and should not be construed as a recommendation to purchase or sell any securities. The information provided regarding such sectors is not a sufficient basis upon which to make an investment decision. Investors seeking financial advice regarding the appropriateness of investing in any securities or investment strategies discussed should consult their financial professional. The Fund’s top five sector holdings (as a percentage of net assets) as of November 30, 2020 were: Financials (31.0%), Energy (15.0%), Health Care (12.2%), Communication Services (12.0%) and Industrials (9.5%). The Fund’s portfolio composition is subject to change at any time.

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 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.

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Fund overview (cont’d)

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Fund at a glance^† (unaudited)

Investment breakdown (%) as a percent of total investments

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Schedule of investments

November 30, 2020

See Notes to Financial Statements.

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