United States Treasury securities, also called Treasuries or Treasurys, are government debt instruments issued by the United States Department of the Treasury to finance government spending as a supplement to taxation. Since 2012, the U.S. government debt has been managed by the Bureau of the Fiscal Service, succeeding the Bureau of the Public Debt.

There are four types of marketable Treasury securities: Treasury bills, Treasury notes, Treasury bonds, and Treasury Inflation Protected Securities (TIPS). The government sells these securities in auctions conducted by the Federal Reserve Bank of New York, after which they can be traded in secondary markets. Non-marketable securities include savings bonds, issued to individuals; the State and Local Government Series (SLGS), purchaseable only with the proceeds of state and municipal bond sales; and the Government Account Series, purchased by units of the federal government.

Treasury securities are backed by the full faith and credit of the United States, meaning that the government promises to raise money by any legally available means to repay them. Although the United States is a sovereign power and may default without recourse, its strong record of repayment has given Treasury securities a reputation as one of the world's lowest-risk investments. This low risk gives Treasuries a unique place in the financial system, where they are used as cash equivalents by institutions, corporations, and wealthy investors.

To finance the costs of World War I, the U.S. government increased income taxes (see the War Revenue Act of 1917) and issued government debt, called war bonds. Traditionally, the government borrowed from other countries, but there were no other countries from which to borrow in 1917.

The Treasury raised funding throughout the war by selling $21.5 billion in Liberty bonds. These bonds were sold at subscription, where officials created coupon price and then sold it at par value. At this price, subscriptions could be filled in as little as one day, but usually remained open for several weeks, depending on demand for the bond.

After the war, the Liberty bonds were reaching maturity, but the Treasury was unable to pay each down fully with only limited budget surpluses. To solve this problem, the Treasury refinanced the debt with variable short and medium-term maturities. Again, the Treasury issued debt through fixed-price subscription, where both the coupon and the price of the debt were dictated by the Treasury.

The problems with debt issuance became apparent in the late 1920s. The system suffered from chronic over-subscription, where interest rates were so attractive that there were more purchasers of debt than required by the government. This indicated that the government was paying too much for debt. As government debt was undervalued, debt purchasers could buy from the government and immediately sell to another market participant at a higher price.

In 1929, the US Treasury shifted from the fixed-price subscription system to a system of auctioning where Treasury bills would be sold to the highest bidder. Securities were then issued on a pro rata system where securities would be allocated to the highest bidder until their demand was full. If more treasuries were supplied by the government, they would then be allocated to the next highest bidder. This system allowed the market, rather than the government, to set the price. On December 10, 1929, the Treasury issued its first auction. The result was the issuing of $224 million three-month bills. The highest bid was at 99.310, with the lowest bid accepted at 99.152.