<p>This study aims to investigate the heterogeneity and function of bone marrow T cells in postmenopausal osteoporosis (PMOP). Single-cell transcriptome sequencing was employed to cluster T cells from the femoral bone marrow of mice with PMOP. A subgroup of T cell closely related to postmenopausal osteoporosis was identified. This T-cell subgroup, which highly expresses CD8a, GZMK, CD6, and fewer expresses GZMA, was defined as CD8a + GZMK + T cells. Flow cytometry and analysis of mice femur samples showed a negative correlation between bone loss and the CD8a + GZMK + T-cell population. Tartrate-resistant acid phosphatase (TRAP) staining and immunofluorescence staining showed a marked reduction in both quantity and fusion of osteoclasts in primary bone marrow macrophages (BMMs) co-cultured with CD8a + GZMK + T cells. qPCR analysis revealed lower expression of genes—NFATC1, CTSK, TRAP, MMP9, and FOS— related to osteoclastogenesis. Similar inhibitory effects on osteoclastogenesis were observed in vitro with treatment using recombinant Granzyme K (GzmK), with effects intensifying as the GzmK concentration increased. RNA sequencing and Western blot analyses revealed significant suppression of the MAPK signaling pathway. The MAPK agonist could partially counteract the inhibitory effects of GzmK on osteoclastogenesis. This study elucidated the heterogeneity of T cells during the pathological process of PMOP. We identified and characterized a T-cell subset, CD8a + GZMK + T cells, which secrete GzmK and inhibit osteoclastogenesis via the P38-MAPK pathway. These findings provide new insights into the role of T cells in the pathology of PMOP and suggest potential therapeutic targets for its treatment.</p>