消除反应（又称脱去反应或消去反应），是一种有机反应。是指一有机化合物分子和其他物质反应，失去部份原子或官能基（称为离去基）。反应后的分子会产生多键，为不饱和有机化合物。

消除反应分为下列两种：

离去基所接的碳为α碳，其上的氢为α氢，而隔壁相邻接的碳及氢则为β碳及β氢。化合物会失去β氢原子的称为 β脱去反应，会失去α氢原子的称为α脱去反应。

An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 reaction. The numbers do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively. In rare cases, for molecules possessing particularly poor leaving groups, a third type of reaction, E1_CB exists.