Moody's Ratings is the credit ratings division of Moody's Corporation. It was known as Moody's Investors Service until March 2024, when the unit was rebranded as Moody's Ratings. Moody's Ratings provides international financial research on bonds issued by commercial and government entities. Along with Standard & Poor's and Fitch Group, Moody's is one of the Big Three credit rating agencies.

The company ranks the creditworthiness of borrowers using a standardized ratings scale which measures expected investor loss in the event of default. Moody's Ratings rates debt securities in several bond market segments. These include government, municipal and corporate bonds; managed investments such as money market funds and fixed-income funds; financial institutions including banks and non-bank finance companies; and asset classes in structured finance. In Moody's Ratings system, securities are assigned a rating from Aaa to C, with Aaa being the highest quality and C the lowest quality.

Moody's was founded by John Moody in 1909, to produce manuals of statistics related to stocks and bonds and bond ratings. In 1975, the company was identified as a Nationally Recognized Statistical Rating Organization (NRSRO) by the U.S. Securities and Exchange Commission. Following several decades of ownership by Dun & Bradstreet, Moody's Investors Service became a separate company in 2000. Moody's Corporation was established as a holding company. On March 6, 2024, Moody's Investors Service was renamed to Moody's Ratings.

Together, Moody's, S&P and Fitch are sometimes referred to as the Big Three credit rating agencies. While credit rating agencies are sometimes viewed as interchangeable, Moody's, S&P and Fitch have different methodologies. All three operate worldwide, maintaining offices on six continents, and rating tens of trillions of dollars in securities.

Moody's Ratings and its close competitors play a key role in global capital markets as supplementary, third-party providers of credit analysis used by banks and other investors when assessing the credit risk of particular securities. This form of third party analysis is particularly useful for smaller and less sophisticated investors, as well as for all investors to use as an external comparison for their own judgments.

Credit rating agencies also play an important role in the laws and regulations of the United States and several other countries, such as those of the European Union. In the United States their credit ratings are used in regulation by the U.S. Securities and Exchange Commission as Nationally Recognized Statistical Rating Organizations (NRSROs) for a variety of regulatory purposes. Among the effects of regulatory use was to enable lower-rated companies to sell bond debt for the first time; their lower ratings merely distinguished them from higher-rated companies, rather than excluding them altogether, as had been the case. However, another aspect of mechanical use of ratings by regulatory agencies has been to reinforce "pro-cyclical" and "cliff effects" of downgrades. In October 2010, the Financial Stability Board (FSB) created a set of "principles to reduce reliance" on credit rating agencies in the laws, regulations and market practices of G-20 member countries. Since the early 1990s, the SEC has also used NRSRO ratings in measuring the commercial paper held by money market funds.

The SEC has designated seven other firms as NRSROs, including, for example, A. M. Best, which focuses on obligations of insurance companies. Companies with which Moody's competes in specific areas include investment research company Morningstar, Inc. and publishers of financial information for investors such as Thomson Reuters and Bloomberg L.P.

Especially since the early 2000s, Moody's frequently makes its analysts available to journalists, and issues regular public statements on credit conditions. Moody's, like S&P, organizes public seminars to educate first-time securities issuers on the information it uses to analyze debt securities.