Bone marrow mesenchymal stem cells (BMMSCs) have been used to treat a variety of autoimmune diseases in clinics. However, the therapeutic effects are largely dependent on the immunomodulatory capacity of culture-expanded BMMSCs. In the present study, we show that aspirin (acetylsalicylic acid, ASA)-treated BMMSCs have significantly improved immunomodulatory function, as indicated by upregulation of regulatory T cells (Tregs) and downregulation of Th17 cells via the 15d-PGJ2/PPARγ/TGF-β1 pathway. Furthermore, the therapeutic effect of ASA-pretreated BMMSCs was confirmed in a dextran sodium sulfate-induced experimental colitis mouse model, in which systemic infusion of ASA-pretreated BMMSCs significantly ameliorated disease activity index and colonic inflammation, along with an increased number of Tregs and decreased number of Th17 cells. Taken together, our results suggest that aspirin treatment is a feasible strategy to promote BMMSC-based immunomodulation.