In enzymology, a spermidine dehydrogenase (EC 1.5.99.6) is an enzyme that catalyzes the chemical reaction

spermidine + acceptor + H₂O ${\displaystyle \rightleftharpoons }$ propane-1,3-diamine + 4-aminobutanal + reduced acceptor

The 3 substrates of this enzyme are spermidine, acceptor, and H₂O, whereas its 3 products are propane-1,3-diamine, 4-aminobutanal, and reduced acceptor.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-NH group of donor with other acceptors. The systematic name of this enzyme class is spermidine:acceptor oxidoreductase. This enzyme is also called spermidine:(acceptor) oxidoreductase. This enzyme participates in urea cycle and metabolism of amino groups and beta-alanine metabolism. It has 2 cofactors: FAD, and Heme.