热质说（Caloric theory）是种错误和受局限的科学理论，曾用来解释热的物理现象。此理论认为热是一种称为「热质」（caloric）的物质，热质是一种无质量的气体，物体吸收热质后温度会升高，热质会由温度高的物体流到温度低的物体，也可以穿过固体或液体的孔隙中。热质说在拉瓦锡1772年用实验推翻燃素说后开始盛行，拉瓦锡的《化学基础》一书就把热列在基本物质之中。

热质说可以解释一些热的现象，不过无法解释一些只要持续作功就可以持续产生热的现象（如摩擦生热）。19世纪中，热质说被机械能守恒所取代；之后，热质说仍然在许多科学文献中出现，一直到19世纪末才消失。目前常用的热量单位卡路里（Calorie）即起源自热质（caloric）。

The caloric theory is an obsolete scientific theory that heat consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores in solids and liquids. The "caloric theory" was superseded by the mid-19th century in favor of the mechanical theory of heat, but nevertheless persisted in some scientific literature -- particularly in more popular treatments -- until the end of the 19th century.