劣币驱逐良币（英语：Bad money drives out good），也称作格雷欣法则或格勒善定律（Gresham's Law），是一个经济学定律。托马斯·格雷欣为16世纪的英国伊丽莎白铸币局长，格雷欣观察到，消费者保留保存成色高的货币（undebased money，贵金属含量高），在市面使用成色低的货币（debased money）进行市场交易，而使得在民间流通的大多为劣币，良币则较少见于世，此定理也被广泛用于非经济学的层面。

In economics, Gresham's law is a monetary principle stating that "bad money drives out good". For example, if there are two forms of commodity money in circulation, which are accepted by law as having similar face value, the more valuable commodity will disappear from circulation.