Obesity elicits immune cell infiltration of adipose tissue provoking chronic low-grade inflammation. Regulatory T cells (Tregs) are specifically reduced in adipose tissue of obese animals. Since interleukin (IL)-21 plays an important role in inducing and maintaining immune-mediated chronic inflammatory processes and negatively regulates Treg differentiation/activity, we hypothesized that it could play a role in obesity-induced insulin resistance. We found IL-21 and IL-21R mRNA expression upregulated in adipose tissue of high-fat diet (HFD) wild-type (WT) mice and in stromal vascular fraction from human obese subjects in parallel to macrophage and inflammatory markers. Interestingly, a larger infiltration of Treg cells was seen in the adipose tissue of IL-21 knockout (KO) mice compared with WT animals fed both normal diet and HFD. In a context of diet-induced obesity, IL-21 KO mice, compared with WT animals, exhibited lower body weight, improved insulin sensitivity, and decreased adipose and hepatic inflammation. This metabolic phenotype is accompanied by a higher induction of interferon regulatory factor 4 (IRF4), a transcriptional regulator of fasting lipolysis in adipose tissue. Our data suggest that IL-21 exerts negative regulation on IRF4 and Treg activity, developing and maintaining adipose tissue inflammation in the obesity state.