Broadly defined, an infidelity clause is a provision in a prenuptial or postnuptial agreement (collectively, "marital agreements") that stipulates that if one party is unfaithful during the marriage, he or she will suffer a financial penalty.

In popular media, infidelity clauses have been classed as a subset of "lifestyle clauses," or clauses that dictate how parties should live during the marriage. Courts have viewed infidelity clauses more starkly, recognizing them as merely one type of provision which aims to punish one party for perceived misconduct during the marriage ("misconduct clauses").