In enzymology, a phenylacetaldehyde dehydrogenase (EC 1.2.1.39) is an enzyme that catalyzes the chemical reaction

phenylacetaldehyde + NAD⁺ + H₂O ${\displaystyle \rightleftharpoons }$ phenylacetate + NADH + 2 H⁺

The 3 substrates of this enzyme are phenylacetaldehyde, NAD⁺, and H₂O, whereas its 3 products are phenylacetate, NADH, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the aldehyde or oxo group of donor with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is phenylacetaldehyde:NAD+ oxidoreductase. This enzyme participates in phenylalanine metabolism and styrene degradation.