Mitochondrial fatty acid synthesis (FAS) generates the octanoyl-group that is required for the synthesis of lipoic acid and is linked to mitochondrial RNA metabolism. All of the human enzymes involved in mitochondrial FAS have been characterized except for beta-ketoacyl thioester reductase (HsKAR), which catalyzes the second step in the pathway. We report here the unexpected finding that a heterotetramer composed of human 17beta-hydroxysteroid dehydrogenase type 8 (Hs17beta-HSD8) and human carbonyl reductase type 4 (HsCBR4) forms the long-sought HsKAR. Both proteins share sequence similarities to the yeast 3-oxoacyl-(acyl carrier protein) reductase (Oar1p) and the bacterial FabG, although HsKAR is NADH dependent, whereas FabG and Oar1p are NADPH dependent. Hs17beta-HSD8 and HsCBR4 show a strong genetic interaction in vivo in yeast, where, only if they are expressed together, they rescue the respiratory deficiency and restore the lipoic acid content of oar1Delta cells. Moreover, these two proteins display a stable physical interaction and form an active heterotetramer. Both Hs17beta-HSD8 and HsCBR4 are targeted to mitochondria in vivo in cultured HeLa cells. Notably, 17beta-HSD8 was previously classified as a steroid-metabolizing enzyme, but our data suggest that 17beta-HSD8 is primarily involved in mitochondrial FAS.