固氮生物（英语：Diazotroph）多为细菌及古菌，能将空气中的氮气固定为较有用的形式，例如︰氨。

固氮生物是能不透过外在资源而固氮的有机体。举例来说，这样的有机体包含︰根瘤菌及属于放射菌的弗兰克氏菌。 所有的固氮生物都有铁钼或铁钒蛋白的固氮酶系统。其中做最多研究的是克雷伯氏肺炎菌（Klebsiella pneumonia）和Azotobacter vinelandii。它们能被广泛利用是因为它们的基因易于培养及生长快速。

Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia.

A diazotroph is an organism that is able to grow without external sources of fixed nitrogen. Examples of organisms that do this are rhizobia and Frankia (in symbiosis) and Azospirillum. All diazotrophs contain iron-molybdenum or -vanadium nitrogenase systems. Two of the most studied systems are those of Klebsiella pneumoniae and Azotobacter vinelandii. These systems are used because of their genetic tractability and their fast growth.