In enzymology, an anthocyanidin reductase (EC 1.3.1.77) is an enzyme that catalyzes the chemical reaction

a flavan-3-ol + 2 NAD(P)+ ${\displaystyle \rightleftharpoons }$ an anthocyanidin + 2 NAD(P)H + H⁺

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The 3 substrates of this enzyme are flavan-3-ol, NAD⁺, and NADP⁺, whereas its 4 products are anthocyanidin, NADH, NADPH, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-CH group of donor with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is flavan-3-ol:NAD(P)+ oxidoreductase. Other names in common use include AtANR, and MtANR. This enzyme participates in flavonoid biosynthesis.