Dihydroorotate dehydrogenase (fumarate) (, dihydroorotate oxidase, pyr4 (gene)) is an enzyme with systematic name (S)-dihydroorotate:fumarate oxidoreductase.
The enzyme is found in the bacterium Lactococcus lactis and the yeast Saccharomyces cerevisiae. It contains the cofactor, flavin mononucleotide and catalyses the following chemical reaction which is part of the biosynthesis of pyrimidine nucleotides.