顺反异构（英文：Cis-trans isomerism），旧称几何异构，是存在于某些双键化合物或环状化合物中的一种立体异构现象。由于存在双键或环，这些分子的自由旋转受阻，产生两个物理性质或化学性质均不相同的异构体，分别称为顺式（cis）和反式（trans）异构体。

“顺式加成”指的是从双键/三键的同一侧进行加成；“反式加成”指的是从双键/三键的两侧进行加成。

在双键化合物中，若与两个双键原子相连的相同或相似的基团处在双键的同侧，则该化合物被称为“顺式”异构体；若两个基团处于异侧，则定义为“反式”异构体，比如右图所示的2-丁烯的两个异构体。

Cis/trans isomerism, also known as geometric isomerism or configurational isomerism, is a term used in organic chemistry to refer to the stereoisomerism engendered in the relative orientation of functional groups within a molecule. It is not to be confused with E/Z isomerism, which is an absolute stereochemical description, and only to be used with alkenes. In general, such isomers contain double bonds that cannot rotate, or they may contain ring structures, where the rotation of bonds is restricted or eliminated. Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes. Cis and trans descriptors are not used for cases of conformational isomerism where the two geometric forms easily interconvert, such as most open-chain single-bonded structures; instead, the terms “syn” and “anti” would be used.