Human d-3-phosphoglycerate dehydrogenase (PHGDH), the first enzyme in the serine biosynthetic pathway, is genomically amplified in tumors including breast cancer and melanoma. In PHGDH-amplified cancer cells, knockdown of PHGDH is not fully rescued by exogenous serine, suggesting possible additional growth-promoting roles for the enzyme. Here we show that, in addition to catalyzing oxidation of 3-phosphoglycerate, PHGDH catalyzes NADH-dependent reduction of α-ketoglutarate (AKG) to the oncometabolite d-2-hydroxyglutarate (d-2HG). Knockdown of PHGDH decreased cellular 2HG by approximately 50% in the PHGDH-amplified breast cancer cell lines MDA-MB-468 (normal concentration 93 μM) and BT-20 (normal concentration 35 μM) and overexpression of PHGDH increased cellular 2HG by over 2-fold in non-PHGDH-amplified MDA-MB-231 breast cancer cells, which normally display very low PHGDH expression. The reduced 2HG level in PHGDH knockdown cell lines can be rescued by PHGDH re-expression, but not by a catalytically inactive PHGDH mutant. The initial connection between cancer and d-2HG involved production of high levels of d-2HG by mutant isocitrate dehydrogenase. More recently, however, elevated d-2HG has been observed in breast cancer tumors without isocitrate dehydrogenase mutation. Our results suggest that PHGDH is one source of this d-2HG.