反应中间体（或中间体/反应中间物）是化学反应中的中间产物。存在时间短，能量高，高反应性的分子．当反应产生时会快速的转变成更稳定的分子．只有在特殊情况下才能将其分离出来并保存，比如低温．中间体的存在能够帮助解释部分化学反应的如何进行．

多数化学反应为多步反应，即反应不会一次完成．且反应中间体高能量，不稳定，容易再反应变成产物．下列反应中的物种X*便是反应中间体：

自由基

碳烯

碳正离子

碳负离子

卡拜

苯炔

IUPAC金色书对“反应中间体”的定义是：寿命长于分子振动的分子实体（原子、离子、分子等），它由反应物（直接或非直接）生成，并且继续（直接或非直接）反应得到产物。

A reaction intermediate or an intermediate is a molecular entity that is formed from the reactants (or preceding intermediates) and reacts further to give the directly observed products of a chemical reaction. Most chemical reactions are stepwise, that is they take more than one elementary step to complete. An intermediate is the reaction product of each of these steps, except for the last one, which forms the final product. Reactive intermediates are usually short lived and are very seldom isolated. Also, owing to the short lifetime, they do not remain in the product mixture.