关系从句（英语：relative clause）或关系子句，是指一类由关系词（英语：relative word）引导的从句。因为这类从句的句法功能多是做定语，所以曾被称为定语从句（英语：attributive clause）。值得注意的是，这类从句除了可以做定语之外，还可以充当状语等其他成分，所以现代语言学多使用“关系从句”这一术语。

在英语中，关系从句主要通过关系代名词法构成。

Relative clauses in the English language are formed principally by means of relative pronouns. The basic relative pronouns are who, which, and that; who also has the derived forms whom and whose. Various grammatical rules and style guides determine which relative pronouns may be suitable in various situations, especially for formal settings. In some cases the relative pronoun may be omitted and merely implied ("This is the man [that] I saw", or "This is the putter he wins with").