质量作用定律（英语：Law of mass action）是化学领域的概念。定律包含两个方向：

两种方向都源自于G.W.Guldberg与P.Waage (1864-1879)的研究，他们的研究是关于利用<动力学数据>和他们所提出的速率式来导出平衡常数。与此同时他们也得以基本确认化学平衡是一种动态的平衡，在这种进行的、变化的平衡中，正向和逆向的反应速率是相等的。

In chemistry, the law of mass action is a mathematical model that explains and predicts behaviours of solutions in dynamic equilibrium. Simply put, it states that the rate of a chemical reaction is proportional to the product of the masses of the reactants. Necessarily, this implies that for a chemical reaction mixture that is in equilibrium, the ratio between the concentration of reactants and products is constant.