附买回协议（repurchase agreement、repo，也称为附买回协定、附买回交易，回购协议或正回购）是货币市场的一类金融商品。在附买回交易中交易双方同意债券持有人卖出债券后在未来约定日期回购债券。回购价格必须高于卖出价格，两者之间的差异由回购利率（repo rate）决定。原先购入债券的一方为贷款人，卖出债券的一方为借款人，利用其债券作担保获取固定利率担保贷款。

附买回协议相当于现货销售和远期合约的组合。在现货销售中贷款人向借款人提供货币，债券则被合法转让给贷款人作为交换。远期合约保证了借款人付清贷款以及贷款人退还债券。远期价格和现货价格之间的差异即贷款利息，而远期合约的结算日期即附买回交易的到期日。

A sale (and) repurchase agreement, also known as a (currency) repo, RP, or sale and repurchase agreement, is a transaction concluded on a deal date t_D between two parties A and B:

(i) A will on the near date t_N sell a specified security S at an agreed price P_N to B