An unenforceable contract or transaction is one that is valid but one the court will not enforce. Unenforceable is usually used in contradiction to void (or void ab initio) and voidable. If the parties perform the agreement, it will be valid, but the court will not compel them if they do not.

An "agreement to agree", where a purported contract contains an obligation to enter into a subsequent agreement in the future, the terms of which are not certain at the time of the initial agreement, is generally considered to lack sufficient certainty to constitute a legally enforceable contract and is therefore unenforceable. However, an agreement under which "the parties contemplate entering into a further, more formal, agreement later" may be enforceable.