自燃（英语：Spontaneous combustion 或 pyrophoric）是指可燃物质在没有外部火花、火焰等火源的作用下，因受热或自身发热并蓄热所产生的自行燃烧。蓄积的热量达到某个温度时，干草、堆肥、煤、亚麻籽油和某些化学物质都有可能发生自燃，这一温度就称作自燃温度。还有一些未经证实的报道称，有人体自燃现象。原因可能是因为衣物等被引燃后波及人体脂肪所导致的燃烧现象，被称为灯芯效应，而不属于严格意义上的自燃。

A pyrophoric substance (from Greek πυροφόρος, pyrophoros, "fire-bearing") ignites spontaneously in air at or below 55 °C (130 °F). Examples are iron sulfide and many reactive metals including uranium, when powdered or thinly sliced. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of argon or (with a few exceptions) nitrogen. Most pyrophoric fires should be extinguished with a Class D fire extinguisher for burning metals.