内生孢子（英语：endospore，又称芽孢或内孢子）是某些特殊种群的细菌，主要是芽孢杆菌属Bacillus与梭菌属Clostridium中细菌产生的特殊休眠体，这些细菌会在缺乏养分的环境时开始进入此休眠状态，有极强的抗逆性，对热、碱、酸、高渗以及辐射均有强耐受性。通常使用的杀灭芽孢的方法为高压灭菌。普通的巴斯德消毒法无法杀灭芽孢。

这些孢子里面含有细菌的遗传物质、核糖体和大量的吡啶二羧酸(Dipicolinic Acid)，吡啶二羧酸是一种内生孢子特有的化学物质，作用是帮助维持孢子处于休眠状态，在孢子里占了高至10%的干重。

An endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form (endo means within), but it is not a true spore (i.e., not an offspring). It is a stripped-down, dormant form to which the bacterium can reduce itself. Endospore formation is usually triggered by a lack of nutrients, and usually occurs in gram-positive bacteria. In endospore formation, the bacterium divides within its cell wall. One side then engulfs the other. Endospores enable bacteria to lie dormant for extended periods, even centuries. Revival of spores millions of years old has been claimed. When the environment becomes more favorable, the endospore can reactivate itself to the vegetative state. Most types of bacteria cannot change to the endospore form. Examples of bacteria that can form endospores include Bacillus and Clostridium.