科斯定理（英语：Coase theorem），描敍一个经济体系内部的资源配置与产出，在外部性存在的情形下，其经济效率所可能受到的影响。这个理论由诺贝尔经济学奖得主罗纳德·科斯在1960年代的论文中提出。乔治·斯蒂格勒在1966年出版的经济学教科书中，首次将他的见解进行归纳，并称为科斯定理。

其核心思想是交易成本。《新帕尔格雷夫经济学词典》中由罗伯特·D.库特对“科斯定理”的解释。他写道：“从强调交易成本解释的角度说，科斯定理可以描述如下：只要交易成本等于零，法定权利（即产权）的初始配置并不影响效率。”

In law and economics, the Coase theorem (pronounced /ˈkoʊs/) describes the economic efficiency of an economic allocation or outcome in the presence of externalities. The theorem states that if trade in an externality is possible and there are sufficiently low transaction costs, bargaining will lead to a Pareto efficient outcome regardless of the initial allocation of property. In practice, obstacles to bargaining or poorly defined property rights can prevent Coasian bargaining. This "theorem" is commonly attributed to Nobel Prize laureate Ronald Coase during his tenure at the University of Chicago. However, Coase himself stated that the theorem was based on perhaps four pages of his 1960 paper "The Problem of Social Cost", and that the "Coase theorem" is not about his work at all.