Abstract Erythrocytosis, an increase in absolute red cell mass, is mainly driven by erythropoietin, while hypoxia-inducible factor (HIF) regulates the expression of a number of genes involved in it, including erythropoietin. Mutations in HIF prolyl 4-hydroxylase 2 (HIF-P4H-2/PHD2/EGLN1), the major regulator of the stability of HIFα subunits, are found in familiar erythrocytosis, and large-spectrum conditional inactivation of HIF-P4H-2 in mice leads to severe erythrocytosis and premature death. Although bone marrow is the primary site for erythropoiesis, spleen retains a capability for extramedullary erythropoiesis. We studied HIF-P4H-2 hypomorphic mice (Hif-p4h-2gt/gt) which show slightly induced erythropoiesis only upon aging despite no increased erythropoietin levels. Spleen was identified as the site of extramedullary erythropoiesis in these mice. Hematopoietic stem cells (HSCs) from spleens of the Hif-p4h-2gt/gt mice showed increased growth of BFU-Es and the mice were protected against anemia by induced extramedullary erythropoiesis. HIF-1α and HIF-2α were stabilized in the spleens, while the Notch ligands and target Jag1, Jag2, Dll1 and Hes1 became downregulated upon aging dependent on HIF-2α. Inhibition of Notch signaling in wild-type spleen HSCs phenocopied the increased growth of BFU-Es in the Hif-p4h-2gt/gt mice. We conclude that HIFα stabilization can mediate non-erythropoietin-driven extramedullary erythropoiesis in the spleen via altered Notch signaling. Disclosures Myllyharju: FibroGen Inc.: Equity Ownership, Research Funding.