在微观经济学中，边际效用（英语：marginal utility），又译为边际效应，是指每添加（或减少）一个单位的商品或服务，它对商品或服务的收益增加（或减少）的效用，也即是「效用──商品或服务量」图的斜率。经济学通常认为，随着商品或服务的量增加，边际效用将会逐步减少，称为边际效应递减定律。

在这里要注意的是，边际效用是指某种物品的消费量每增加一单位所增加的满足程度，针对的是消费者；而边际报酬（marginal returns）则对应生产者。同样，随着商品或服务的量增加，边际报酬也将会逐步减少，称为边际报酬递减定律。

In economics, the marginal utility of a good or service is the gain from an increase, or loss from a decrease, in the consumption of that good or service. Economists sometimes speak of a law of diminishing marginal utility, meaning that the first unit of consumption of a good or service yields more utility than the second and subsequent units, with a continuing reduction for greater amounts. The marginal decision rule states that a good or service should be consumed at a quantity at which the marginal utility is equal to the marginal cost.