In enzymology, a nitrite reductase (NO-forming) (EC 1.7.2.1) is an enzyme that catalyzes the chemical reaction The 3 substrates of this enzyme are nitric oxide, H2O, and ferricytochrome c, whereas its 3 products are nitrite, ferrocytochrome c, and H+.

This enzyme belongs to the family of oxidoreductases, specifically those acting on other nitrogenous compounds as donors with a cytochrome as acceptor.

The systematic name of this enzyme class is nitric-oxide:ferricytochrome-c oxidoreductase.

Other names in common use include cd-cytochrome nitrite reductase, [nitrite reductase (cytochrome)] [misleading, see comments.

As of late 2007, 20 structures have been solved for this class of enzymes, with PDB accession codes 1RZP, 1RZQ, 1SJM, 1SNR, 1ZDQ, 1ZDS, 1ZV2, 2A3T, 2AVF, 2B08, 2BW4, 2BW5, 2BWD, 2BWI, 2DV6, 2DWS, 2DWT, 2DY2, 2FJS, and 2JFC.