链反应（或连锁反应、链式反应，Chain reaction}）是指反应的产物或副产物又可作为其他反应的原料，从而使反应反复发生。在化学中，链反应通常指光、热、辐射或引发剂作用下，反应中交替产生活性中间体（如自由原子或自由基），从而使得反应一直进行下去。它是由基元反应组合成的更加复杂的复合反应。

以 这个反应为例，反应中产生的Cl·可以与H₂反应生成H·，而H·又可与Cl₂反应生成Cl·，如此往复进行，使反应一直延续下去。

产生活性中间体的过程称为链引发，活性中间体与反应物分子反复作用生成产物的过程称为链增长或链传递，活性中间体最后湮灭的过程称为链终止。一个活性中间体只能产生一个新的活性中间体的反应称为直链反应，可以产生两个或多个新的活性中间体的反应称为支链反应。（注意与有机化学中的定义相区别！）

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events.

Chain reactions are one way in which systems which are in thermodynamic non-equilibrium can release energy or increase entropy in order to reach a state of higher entropy. For example, a system may not be able to reach a lower energy state by releasing energy into the environment, because it is hindered or prevented in some way from taking the path that will result in the energy release. If a reaction results in a small energy release making way for more energy releases in an expanding chain, then the system will typically collapse explosively until much or all of the stored energy has been released.