固氮酶（英语：Nitrogenase）是一类在许多有机体中被利用于将空气中的氮气转化为含氮化合物的酶。这类酶是现在已被人们发现的唯一一种能完成该过程的酶。氮一般以含有键能较高的氮-氮三键的氮分子形式存在于自然界中，必须将这三个化学键完全破坏才能把该双原子分子中的两个氮原子分开。

固氮酶可以看作是固氮作用中的催化剂，固氮酶使以下反应的活化能降低，从而使反应更容易进行。

固氮酶催化反应的简化反应方程序为：

详细反应方程序为：

反应底物为：

8 铁氧还蛋白_red. + 8 H + N₂ + 16 ATP + 16 H₂O

Nitrogenases are enzymes (EC1.18.6.1EC1.19.6.1) that are produced by certain bacteria, such as cyanobacteria (blue-green algae). These enzymes are responsible for the reduction of nitrogen (N₂) to ammonia (NH₃). Nitrogenases are the only family of enzymes known to catalyze this reaction, which is a key step in the process of nitrogen fixation. Nitrogen fixation is required for all forms of life, with nitrogen being essential for the biosynthesis of molecules (nucleotides, amino acids) that create plants, animals and other organsisms.