CD4(+)Foxp3(+) regulatory T cells (Tregs) in the tumor microenvironment restrain antitumor immunity, resulting in tumor aggression and poor survival in hepatocellular carcinoma (HCC). CD8(+)CD122(+) Tregs have been previously shown to be more potent in immunosuppression than are CD4(+)Foxp3(+) Tregs. Previous studies have demonstrated that resveratrol exerts its anti-cancer effects by downregulating CD4(+)Foxp3(+) and M2-like macrophages, two key immunoregulatory cells that maintain the immunosuppressive tumor microenvironment. In this study, we found that resveratrol inhibited the tumor growth in a subcutaneous Hepa1-6 HCC model and decreased the frequency of CD8(+)CD122(+) Tregs in the tumor as well as lymph nodes and spleen of the tumor-bearing mice. It also increased the percentage of IFN-gamma-expressing CD8(+) T cells in the tumor and peripheral lymphoid organs. The antitumor effects of resveratrol were partially reversed by the adoptive transfer of exogenous CD8(+)CD122(+) Tregs into the tumor-bearing mice. Meanwhile, resveratrol treatment downregulated immunosuppressive cytokines, including TGF-beta 1 and interleukin-10, in the tumor while elevating antitumor cytokines, TNF-alpha and IFN-gamma. It also inhibited the activation of STAT3 signaling in the tumor. As expected, resveratrol reduced the percentage of M2-like macrophages in the mice. Importantly, resveratrol suppressed orthotopic H22 tumor growth and decreased the frequency of CD8(+)CD122(+) Tregs and M2-like macrophages in the tumor-bearing mice. Furthermore, our studies showed that resveratrol, at non-cytotoxic concentrations, inhibited CD8(+)CD122(+) Treg differentiation from CD8(+)CD122(-) T cells in vitro. Thus, our studies unveiled a new immune mechanism underlying the immunosuppressive tumor microenvironment and demonstrated that resveratrol could help reverse it by diminishing CD8(+)CD122(+) Tregs.