Knee osteoarthritis (OA) is the most common cause of musculoskeletal pain and disability in the knee. Though traditionally thought a mechanical wear-and-tear disease, in recent years, knee OA as a low-grade, chronic inflammatory disease has been increasingly recognized. In this study, we examined the Treg responses in non-obese knee OA patients at different stages. Significantly elevated frequencies of CD4⁺CD25⁺Foxp3⁺ Tregs were found in OA patients, while on the other hand, lower IL-10 secretion from Tregs in OA patients was observed. Importantly, this decrease in IL-10 was associated with reduced Tim-3 expression on Tregs. Although both Tim-3⁻ and Tim3⁺ Tregs could secrete IL-10, the majority of IL-10 was observed in Tim-3⁺ Tregs. Reduction of Tim-3⁺ Tregs in OA patients resulted in less IL-10-producing Tregs. Interestingly, the OA patients in more advanced stages showed further reductions in IL-10 and Tim-3 expression. In conclusion, our results revealed an immunoregulatory disorder in OA independent of obesity, and demonstrated a potential mechanism in establishing the proinflammatory status of OA patients.