乘数效应（英语：Multiplier Effect），更完整地说是收入／支出乘数效应，是宏观经济学的一个概念，是指支出的变化导致经济总需求与其不成比例的变化。这一概念通常与凯恩斯经济学相联系，其他一些经济学学派低估或否认这一效应对于宏观经济，尤其是长期看来的重要性。

当一件事情发生时，而去触发事情的原因扩大时，其事情所得之结果也会因而以更大的辐度扩大。例如，当市场好景时，政府投资或公共开支扩大、税收减少时，对国民收入有加倍扩大的作用，从而产生宏观经济的扩张效应。

公式：

此时 k=1/(1-c)，c=边际消费倾向。

In economics, the fiscal multiplier (not to be confused with monetary multiplier) is the ratio of a change in national income to the change in government spending that causes it. More generally, the exogenous spending multiplier is the ratio of a change in national income to any autonomous change in spending (private investment spending, consumer spending, government spending, or spending by foreigners on the country's exports) that causes it. When this multiplier exceeds one, the enhanced effect on national income is called the multiplier effect. The mechanism that can give rise to a multiplier effect is that an initial incremental amount of spending can lead to increased consumption spending, increasing income further and hence further increasing consumption, etc., resulting in an overall increase in national income greater than the initial incremental amount of spending. In other words, an initial change in aggregate demand may cause a change in aggregate output (and hence the aggregate income that it generates) that is a multiple of the initial change.