In enzymology, a 3-methyl-2-oxobutanoate dehydrogenase (EC 1.2.4.4) is an enzyme that catalyzes the chemical reaction

3-methyl-2-oxobutanoate + [dihydrolipoyllysine-residue (2-methylpropanoyl)transferase] lipoyllysine ${\displaystyle \rightleftharpoons }$ [dihydrolipoyllysine-residue (2-methylpropanoyl)transferase] S-(2-methylpropanoyl)dihydrolipoyllysine + CO₂

The 3 substrates of this enzyme are 3-methyl-2-oxobutanoate, dihydrolipoyllysine-residue (2-methylpropanoyl)transferase, and lipoyllysine, whereas its 3 products are dihydrolipoyllysine-residue (2-methylpropanoyl)transferase, S-(2-methylpropanoyl)dihydrolipoyllysine, and CO₂.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the aldehyde or oxo group of donor with a disulfide as acceptor.

This enzyme participates in valine, leucine and isoleucine degradation. It employs one cofactor, thiamin diphosphate. It is the E1 subunit of a catalytic complex.

As of late 2007, twenty-nine structures have been solved for this class of enzymes, with PDB accession codes 1DTW, 1OLS, 1OLU, 1OLX, 1U5B, 1UM9, 1UMB, 1UMC, 1UMD, 1V11, 1V16, 1V1M, 1V1R, 1WCI, 1X7W, 1X7X, 1X7Y, 1X7Z, 1X80, 2BEU, 2BEV, 2BEW, 2BFB, 2BFC, 2BFD, 2BFE, 2BFF, 2BP7, and 2J9F.