Mesenchymal stem cells (MSCs) are multipotent stem cells with the ability to migrate to sites of inflammation and injury, where they participate in tissue regeneration and repair. The present study aimed to investigate the effects of T cell activation and inflammation on the differentiation of MSCs. Human trabecular bone-derived MSCs were isolated from patients undergoing total hip replacement, and T cells were isolated and purified from peripheral blood mononuclear cells (PBMCs) using CD3 MicroBeads. MSCs were co-cultured with activated T cells to mimic the inflammatory microenvironment. MTS assay was used to detect cell proliferation. qRT-PCR, western blotting, histology and immunohistochemical staining were used to detect the adipo-/osteo-specific gene expression and the relative signaling pathway. The MTS results showed that higher concentrations of T cells significantly increased the proliferation of MSCs. Expression of the inflammatory gene IL-6 was upregulated, while expression of IL-10 and INF gamma was downregulated in MSCs exposed to activated T cells. The results also showed that PHA-activated T cells significantly upregulated the expression of PPAR gamma and FABP4 (adipo-specific genes) in MSCs, but no difference was noted in the expression of RUNX2, osteocalcin and ALP (osteo-specific genes) at the protein level. T cell treatment and inflammation inhibited the protein expression of TGF-beta 1 and the phosphorylation of Smad3, resulting in the weakening of the TGF-beta/Smad pathway and enhancing the adipogenic differentiation of MSCs. The results indicated that PHA-activated T cells and inflammation could promote adipogenesis without affecting the late stage of osteogenesis of MSCs, by increasing the expression of key adipogenic genes through TGF-beta/Smad3 signaling.