Bone marrow fibrosis is a critical component of primary myelofibrosis (PMF). However, the origin of the myofibroblasts that drive fibrosis is unknown. Using genetic fate mapping we found that bone marrow leptin receptor (Lepr)-expressing mesenchymal stromal lineage cells expanded extensively and were the fibrogenic cells in PMF. These stromal cells downregulated the expression of key haematopoietic-stem-cell-supporting factors and upregulated genes associated with fibrosis and osteogenesis, indicating fibrogenic conversion. Administration of imatinib or conditional deletion of platelet-derived growth factor receptor a (Pdgfra) from Lepr⁺ stromal cells suppressed their expansion and ameliorated bone marrow fibrosis. Conversely, activation of the PDGFRA pathway in bone marrow Lepr⁺ cells led to expansion of these cells and extramedullary haematopoiesis, features of PMF. Our data identify Lepr⁺ stromal lineage cells as the origin of myofibroblasts in PMF and suggest that targeting PDGFRA signalling could be an effective way to treat bone marrow fibrosis.