In corporate finance, distressed securities are securities over companies or government entities that are experiencing financial or operational distress, default, or are under bankruptcy. As far as debt securities, this is called distressed debt. Purchasing or holding such distressed-debt creates significant risk due to the possibility that bankruptcy may render such securities worthless (zero recovery).

The deliberate investment in distressed securities as a strategy, while potentially lucrative, has a significant level of risk as the securities may become worthless. To do so requires significant levels of resources and expertise to analyze each investment, the related going concern risk and assess its position in an issuer's capital structure along with the likelihood of ultimate recovery. Distressed securities tend to trade at substantial discounts to their intrinsic or par value and are therefore considered to be below investment grade. This usually limits the number of potential investors to large institutional investors—such as hedge funds, private equity firms, investment banks, and specialist investment firms such as vulture funds.

In 2012, Edward Altman, a professor emeritus at the NYU Stern School of Business, and an expert on bankruptcy theory, estimated that there were "more than 200 financial institutions investing between $350–400 billion in the distressed debt market in the United States".

The market developed for distressed securities as the number of large public companies in financial distress increased in the 1980s and early 1990s. In 1992, professor Edward Altman, who developed the Altman Z-score formula for predicting bankruptcy in 1968, estimated "the market value of the debt securities" of distressed firms as "is approximately $20.5 billion, a $42.6 billion in face value". By 1993 the investment community had become increasingly interested in the potential market for distressed firms' debt. At that time distressed securities "yielded a minimum ten percent over comparable maturity of U.S. Treasury bonds... Adding private debt with public registration rights allows private bank debt and trade claims of defaulted and distressed companies to bring the total book value of defaulted and distressed securities to $284 billion, a market value of $177 billion."

The distressed securities investment strategy exploits the fact many investors are unable to hold securities that are below investment grade.

Some investors have deliberately used distressed debt as an alternative investment, where they buy the debt at a deep discount and aim to realize a high return if the company or country does not go bankrupt or experience defaults. The major buyers of distressed securities are typically large institutional investors, who have access to sophisticated risk management resources such as hedge funds, private equity firms and units of investment banks. Firms that specialize in investing in distressed debt are often referred to as vulture funds.

Investors in distressed securities often try to influence the process by which the issuer restructures its debt, narrows its focus, or implements a plan to turn around its operations. Investors may also invest new capital into a distressed company in the form of debt or equity. According to a 2006 report by Edward Altman, a professor of finance at the NYU Stern School of Business, distressed debt investments earned well above average returns in 2006 and there were more than 170 institutional distressed debt investors. These institutions used "very strong and varied strategies including the traditional passive buy-and-hold and arbitrage plays, direct lending to distressed companies, active-control elements, foreign investing, emerging equity purchases and equity plays during the reorganization of a firm in bankruptcy". The most common distressed securities are bonds and bank debt.

While there is no precise definition, fixed-income instruments with a yield to maturity in excess of 1,000 basis points over the risk-free rate of return (e.g., Treasuries) are commonly thought of as being distressed. Distressed securities often carry ratings of CCC or below from agencies such as Standard & Poor's, Moody's and Fitch.