Histone variants contribute to the complexity of the chromatin landscape and play an integral role in defining DNA domains and regulating gene expression. The histone H3 variant H3.3 is incorporated into genic elements independent of DNA replication by its chaperone HIRA. Here we demonstrate that Hira is required for the self-renewal of adult hematopoietic stem cells (HSCs) and to restrain erythroid differentiation. Deletion of Hira led to rapid depletion of HSCs while differentiated hematopoietic cells remained largely unaffected. Depletion of HSCs after Hira deletion was accompanied by increased expression of bivalent and erythroid genes, which were exacerbated upon cell division and paralleled increased erythroid differentiation. Assessing H3.3 occupancy identified a subset of polycomb-repressed chromatin in HSCs that depend on HIRA to maintain the inaccessible, H3.3-occupied state for gene repression. HIRA-dependent H3.3 incorporation thus defines distinct repressive chromatin that represses erythroid differentiation of HSCs. Overall design: RNA-seq in WT and HIRA KO HSCs and MPPs sorted from adult bone marrow with 4 biological replicates each, and RNA-seq in WT and HIRA KO HSCs and MPPs that are H2B-GFP High or H2B-GFP Low sorted from adult bone marrow with 3 biological replicates each.