Intratumoral hypoxia stimulates enrichment of breast cancer stem cells (BCSC), which are critical for metastasis and patient mortality. Here we report a metabolic adaptation that is required for hypoxia-induced BCSC enrichment and metastasis. Hypoxia-inducible factors coordinately regulate expression of genes encoding phosphoglycerate dehydrogenase (PHGDH) and five downstream enzymes in the serine synthesis pathway and mitochondrial one-carbon (folate) cycle. RNAi-mediated silencing of PHGDH expression in both estrogen receptor-positive and negative breast cancer cells led to decreased NADPH levels, disturbed mitochondrial redox homeostasis, and increased apoptosis, which abrogated BCSC enrichment under hypoxic conditions. PHGDH-deficient cells exhibited increased oxidant levels and apoptosis, as well as loss of BCSC enrichment, in response to treatment with carboplatin or doxorubicin. PHGDH-deficient cells were relatively weakly tumorigenic and tumors that did form were deficient in BCSCs, abolishing metastatic capacity. Our findings highlight a role for PHGDH in the formation of secondary (recurrent or metastatic) tumors, with potential implications for therapeutic targeting of advanced cancers.