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In enzymology, a 3-aci-nitropropanoate oxidase (EC 1.7.3.5) is an enzyme that catalyzes the chemical reaction

3-aci-nitropropanoate + O₂ + H₂O ${\displaystyle \rightleftharpoons }$ 3-oxopropanoate + nitrite + H₂O₂

The 3 substrates of this enzyme are 3-aci-nitropropanoate, O₂, and H₂O, whereas its 3 products are 3-oxopropanoate, nitrite, and H₂O₂.

This enzyme belongs to the family of oxidoreductases, specifically those acting on other nitrogenous compounds as donors with oxygen as acceptor. The systematic name of this enzyme class is 3-aci-nitropropanoate:oxygen oxidoreductase. This enzyme is also called propionate-3-nitronate oxidase. It employs one cofactor, FMN.

• Porter DJ, Bright HJ (1987). "Propionate-3-nitronate oxidase from Penicillium atrovenetum is a flavoprotein which initiates the autoxidation of its substrate by O2". J. Biol. Chem. 262 (30): 14428–34. PMID 3667582.