A repurchase agreement, also known as a repo, RP, or sale and repurchase agreement, is a form of secured short-term borrowing, usually, though not always, using government securities as collateral. A contracting party sells a security to a lender and, by agreement between the two parties, repurchases the security back shortly afterwards, at a slightly higher contracted price. The difference in the prices and the time interval between sale and repurchase creates an effective interest rate on the loan. The mirror transaction, a "reverse repurchase agreement," is a form of secured contracted lending in which a party buys a security along with a concurrent commitment to sell the security back in the future at a specified time and price. Because this form of funding is often used by dealers, the convention is to reference the dealer's position in a transaction with a counterparty. Central banks also use repo and reverse repo transactions to manage banking system reserves. When the Federal Reserve borrows funds to drain reserves, it can do so by selling a government security from its inventory with a commitment to buy it back in the future; it calls the transaction a reverse repo because the dealer counterparty to the Fed is lending money. Similarly, when the Federal Reserve wishes to add to banking reserves, it can buy a government security with a forward commitment to sell it back. It calls this transaction a repo because the Fed counterparty is borrowing money.

The repo market is an important source of funds for large financial institutions in the non-depository banking sector, which has grown to rival the traditional depository banking sector in size. Large institutional investors such as money market mutual funds lend money to financial institutions such as investment banks, in exchange for (or secured by) collateral, such as Treasury bonds and mortgage-backed securities held by the borrower financial institutions. An estimated $1 trillion per day in collateral value is transacted in the U.S. repo markets.

In 2007–2008, a run on the repo market, in which funding for investment banks was either unavailable or at very high interest rates, was a key aspect of the subprime mortgage crisis that led to the Great Recession. During September 2019, the U.S. Federal Reserve intervened in the role of investor to provide funds in the repo markets, when overnight lending rates jumped due to a series of technical factors that had limited the supply of funds available.

In a repo, the investor/lender provides cash to a borrower, with the loan secured by the collateral of the borrower, typically bonds. In the event the borrower defaults, the investor/lender gets the collateral. Investors are typically financial entities such as money market mutual funds, while borrowers are non-depository financial institutions such as investment banks and hedge funds. The investor/lender charges interest (the repo rate), which together with the principal is repaid on repurchase of the security as agreed.

A repo is economically similar to a secured loan, with the buyer (effectively the lender or investor) receiving securities for collateral to protect himself against default by the seller. The party who initially sells the securities is effectively the borrower. Many types of institutional investors engage in repo transactions, including mutual funds and hedge funds.

Although the transaction is similar to a loan, and its economic effect is similar to a loan, the terminology differs from that applying to loans: the seller legally repurchases the securities from the buyer at the end of the loan term. However, a key aspect of repos is that they are legally recognised as a single transaction (important in the event of counterparty insolvency) and not as a disposal and a repurchase for tax purposes. By structuring the transaction as a sale, a repo provides significant protections to lenders from the normal operation of U.S. bankruptcy laws, such as the automatic stay and avoidance provisions.

Almost any security may be employed in a repo, though highly liquid securities are preferred as they are more easily disposed of in the event of a default and, more importantly, they can be easily obtained in the open market if the buyer has created a short position in the repo security by a reverse repo and market sale; by the same token, non liquid securities are discouraged.

Treasury or Government bills, corporate and Treasury/Government bonds, and stocks may all be used as "collateral" in a repo transaction. Unlike a secured loan, however, legal title to the securities passes from the seller to the buyer. Coupons (interest payable to the owner of the securities) falling due while the repo buyer owns the securities are, in fact, usually passed directly onto the repo seller. This might seem counter-intuitive, as the legal ownership of the collateral rests with the buyer during the repo agreement. The agreement might instead provide that the buyer receives the coupon, with the cash payable on repurchase being adjusted to compensate, though this is more typical of sell/buybacks.