不当得利是民事实体法中债法上的重要概念，意指无法律上的正当原因，而受有法律上权利或利益，通常是民事实体法的请求权基础之一。

不当得利的型态主要有两种：给付型不当得利和非给付型不当得利。给付型不当得利，指基于给付行为而发生的不当得利；例如原本基于买卖契约应给予他人物品，之后契约失效，该给付之法律上原因不再存在的情形。非给付型不当得利，指并非基于给付行为而产生的情形；例如某人将他人所有之油漆取来漆在自家的墙壁上，该油漆之利益亦属不当得利，惟其并非因给付行为而来。

在不当得利的情形，利益受损害者得向获益者请求利益返还。

Unjust enrichment is a legal concept referring to situations in which one person is enriched at the expense of another in circumstances which the law treats as unjust. Where an individual is unjustly enriched, the law imposes an obligation upon the recipient to make restitution, subject to defences such as change of position. Liability for an unjust (or unjustified) enrichment arises irrespective of wrongdoing on the part of the recipient. The concept of unjust enrichment can be traced to Roman law and the maxim that "no one should be benefited at another's expense": nemo locupletari potest aliena iactura or nemo locupletari debet cum aliena iactura.