环加成反应（英文：Cycloaddition）是两个或多个不饱和化合物（或同一化合物的不同部分）结合生成环状加合物，并伴随有系统总键级数减少的化学反应。它可以是周环反应或非协同的分步反应。逆过程称为环消除反应。

环加成反应的两种主要类型是狄尔斯-阿尔德反应和1,3-偶极环加成反应。

根据前线轨道理论，两个分子之间的环加成反应符合以下几点：

A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction. Many but not all cycloadditions are concerted. As a class of addition reaction, cycloadditions permit carbon–carbon bond formation without the use of a nucleophile or electrophile.