In enzymology, a nitrite reductase [NAD(P)H] (EC 1.7.1.4) is an enzyme that catalyzes the chemical reaction

ammonium hydroxide + 3 NAD(P)⁺ + H₂O ${\displaystyle \rightleftharpoons }$ nitrite + 3 NAD(P)H + 3 H⁺

The 4 substrates of this enzyme are ammonium hydroxide, NAD⁺, NADP⁺, and H₂O, whereas its 4 products are nitrite, NADH, NADPH, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on other nitrogenous compounds as donors with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is ammonium-hydroxide:NAD(P)+ oxidoreductase. Other names in common use include nitrite reductase (reduced nicotinamide adenine dinucleotide, (phosphate)), NADH-nitrite oxidoreductase, NADPH-nitrite reductase, assimilatory nitrite reductase, nitrite reductase [NAD(P)H2], and NAD(P)H2:nitrite oxidoreductase. This enzyme participates in nitrogen metabolism. It has 3 cofactors: FAD, Iron, and Siroheme.

As of late 2007, only one structure has been solved for this class of enzymes, with the PDB accession code 2JO6.

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