In civil law systems, a synallagmatic contract is a contract in which each party to the contract is bound to provide something to the other party.^[1] Its name is derived from the Ancient Greek συνάλλαγμα (synallagma), meaning mutual agreement.^[2] Examples of synallagmatic contracts include contracts of sale, of service, or of hiring.

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In common law jurisdictions, it is roughly the equivalent of a bilateral contract and may be contrasted with a gift (as such a relationship is not one of contract) or a unilateral contract in which only one party makes an enforceable promise.

In his comments on the case of Hong Kong Fir Shipping Co Ltd. v Kawasaki Kisen Kaisha Ltd. (1957), Lord Diplock said:

"Every synallagmatic contract contains in it the seeds of the problem - in what event will a party be relieved of his undertaking to do that which he has agreed to do but has not yet done?" ^[3]