A Treasury Bill, often abbreviated as T-Bill, is a type of security issued by the United States Treasury through the Bureau of Public Debt. Along with an assortment of other securities, T-Bills are used to finance the United States Government by borrowing money from citizens. Investors purchase T-Bills when they become available, and when they mature after a set period of time, usually less than a year, the investors may redeem their T-Bills for the face value. The purchase price of the T-Bill serves as a temporary loan to the United States Government, which returns it when the T-Bill matures.

The smallest face value for a T-Bill is $1,000 US Dollars (USD). The T-Bill is sold at a discount, which is determined by the Bureau of Public Debt, but the Treasury pays the full face value when it is redeemed. For example, an investor might purchase a 90-day T-Bill for $900 USD, and earn a $100 USD return on the investment when the T-Bill is redeemed. Unlike many other securities, a T-Bill does not bear interest, but the return on a T-Bill is highly predictable and very stable, barring complete financial collapse of the United States Treasury.

Investors may choose to include T-Bills in their profiles because they are highly stable investments with a pre-set time to maturity and a dependable return. Unlike more risky investments, a T-Bill is unlikely to return a substantial sum, but when they are traded on large volume, they can represent a substantial return. Investors can potentially purchase millions of dollars worth of T-Bills, assuming that they possess the available capital. They are also extremely liquid assets, making them a versatile and useful addition to a diverse investment portfolio.

While private investors can and do purchase T-Bills, banks and other financial institutions are capable of purchasing them on a much larger scale, and thus make up the bulk of the trade in T-Bills on the day of the initial offering. Once purchased from the Treasury, a T-Bill can be sold or traded before it matures and is ready to be redeemed, and many individuals purchase T-Bills on the secondary market, from banks and institutions which purchased the bills from the Treasury. As compared with other Treasury securities, the T-Bill matures much more quickly, creating a rapid turnover investment, as opposed to the Treasury Note, which matures in two to 10 years, or Treasury Bonds, which take 10-30 years to mature.