Bronchiolitis obliterans syndrome (BOS) is a progressive, fatal obstructive lung disease that occurs following lung transplant, where it is termed chronic lung allograft dysfunction BOS (CLAD-BOS), or as the primary manifestation of pulmonary chronic graft versus host disease (cGVHD-BOS) following allogeneic hematopoietic stem cell transplant. Disease pathogenesis is poorly understood; however, chronic alloreactivity is common to both conditions, suggesting a shared pathophysiology. We performed single-cell RNA-Seq (scRNA-Seq) on explanted human lungs from 4 patients with CLAD-BOS, 3 patients with cGVHD-BOS, and 3 deceased controls to identify cell types, genes, and pathways enriched in BOS to better understand disease mechanisms. In both forms of BOS, we found an expanded population of CD8+ tissue resident memory T cells (TRM), which was distinct to BOS compared with other chronic lung diseases. In addition, BOS samples expressed genes and pathways associated with macrophage chemotaxis and proliferation, including in nonimmune cell populations. We also identified dysfunctional stromal cells in BOS, characterized by pro- and antifibrotic gene programs. These data suggest substantial cellular and molecular overlap between CLAD- and cGVHD-BOS and, therefore, common pathways for possible therapeutic intervention.