A treasury (in plural form: treasuries or treasury securities) is a security (debt security) that is issued by a treasury department (e.g., US Treasury). It is an IOU made and issued by the government agency in charge of country-wide financial affairs. For holders/ investors, treasury securities (or simply treasuries) are considered one of the safest investments (if issued by a credit-worthy government) given that principal repayment and interest payment are backed by the full faith and credit of the government.

In the United States, Treasury securities are debt obligations issued by the U.S. Department of the Treasury. Treasury securities are primarily categorized according to their maturity. From shortest to longest maturity, these securities are treasury bills (T-bills), treasury notes (T-notes), and treasury bonds (T-bonds). Investors can hold treasuries until maturity date or sell them before end of maturity.

Other treasuries may be designed to provide for a specific feature that investors seek, especially on mid- to long-term, such as protection against inflation. To that end, treasury inflation protected securities (TIPS) are issued (usually for a term of 5, 10, or 30 years). Unlike standard treasury securities, where the principal is fixed, the principal of a TIPS can change, up and down, over its maturity. When the TIPS matures, if the principal is higher than its original level, the holder will receive the difference. If the principal is equal to or lower than the original amount, the investor will only get the original amount.