在化学动力学中，反应速率常数，又称速率常数 k或 λ是化学反应速率的量化表示方式。

对于反应物A和反应物B反应成生成物C的化学反应，反应速率可表示成此式：

k(T)是反应速率常数，会随温度改变

[X]是假定反应发生处遍布于固定容积的溶液内，物质X的容积摩尔浓度。(当反应发生于一定范围，就能以X的单位面积摩尔数表示)

指数m和n称为反应级数，取决于反应机理。可由实验测定。

若为一次反应，亦可写成以下形式：

E_a是活化能，R是气体常数。因为温度T的分子能量可依波兹曼分布(en:Boltzmann distribution)求得，我们可预知能量大于E_a的碰撞比例随e变化，A 是指数前因子(en:Pre-exponential factor)或频率因子。

In chemical kinetics a reaction rate constant or reaction rate coefficient, k, quantifies the rate of a chemical reaction.

For a reaction between reactants A and B to form product C

the reaction rate is often found to have the form:

Here k(T) is the reaction rate constant that depends on temperature. [A] and [B] are the molar concentrations of substances A and B in moles per unit volume of solution, assuming the reaction is taking place throughout the volume of the solution. (For a reaction taking place at a boundary one would use instead moles of A or B per unit area).