Background: Osteoarthritis (OA) is a degenerative disease characterized by cartilage damage. We aimed to improve the understanding of the protective mechanism of synovial mesenchymal stem cell (SMSC)-derived extracellular vesicles (EVs) in cartilage damage of OA. Methods: SMSCs and SMSC-EVs were isolated from synovial biopsies of patients without OA and then identified. The pathological microenvironment of chondrocytes in OA was simulated by inducing SW1353 cells with interleukin (IL)-1β, followed by SMSC-EV treatment to assess SW1353 cell proliferation, apoptosis and inflammation. Endocytosis of Dil-labeled EVs by SW1353 cells was observed. microRNA (miR)-26a-5p expression in EVs and EV-treated SW1353 cells was assessed. The effect of miR-26a-5p was evaluated after it was down-regulated in SMSCs, followed by extraction of EVs, which acted on SW1353 cells. The target relationship of miR-26a-5p and phosphatase and tensin homologue (PTEN) was predicted and confirmed. The role of PTEN in OA was evaluated after it was overexpressed. Functional assays were implemented in vivo to certify the role of SMSC-EVs in OA. Results: SMSC-EVs enhanced IL-1β-induced SW1353 cell proliferation, whereas they inhibited apoptosis and inflammation. EVs were endocytosed by SW1353 cells and delivered miR-26a-5p into SW1353 cells to overexpress miR-26a-5p. Down-regulation of miR-26a-5p in EVs attenuated the protection of EVs against IL-1β-induced cell damage. miR-26a-5p targeted PTEN, for which overexpression spoiled the protection of EVs against IL-1β-induced cell damage. SMSC-EVs carrying miR-26a-5p repaired cartilage damage of OA. Conclusions: SMSC-EVs carried miR-26a-5p into chondrocytes to up-regulate miR-26a-5p and inhibit PTEN, thereby inhibiting apoptosis and inflammation and ameliorating cartilage damage of OA.Keywords: PTEN; cartilage damage; extracellular vesicles; microRNA-26a-5p; osteoarthritis; synovial mesenchymal stem cells.