IL-21 is a type I cytokine essential for immune cell differentiation and function. Although IL-21 can activate several STAT family transcription factors, previous studies focused mainly on the role of STAT3 in IL-21 signaling. Here, we investigated the role of STAT1 and show that STAT1 and STAT3 have at least partially opposing roles in IL-21 signaling in CD4⁺ T cells. IL-21 induced STAT1 phosphorylation, and this was augmented in Stat3-deficient CD4⁺ T cells. RNA-Seq analysis of CD4⁺ T cells from Stat1- and Stat3-deficient mice revealed that both STAT1 and STAT3 are critical for IL-21–mediated gene regulation. Expression of some genes, including Tbx21 and Ifng, was differentially regulated by STAT1 and STAT3. Moreover, opposing actions of STAT1 and STAT3 on IFN-γ expression in CD4⁺ T cells were demonstrated in vivo during chronic lymphocytic choriomeningitis infection. Finally, IL-21–mediated induction of STAT1 phosphorylation, as well as IFNG and TBX21 expression, were higher in CD4⁺ T cells from patients with autosomal dominant hyper-IgE syndrome, which is caused by STAT3 deficiency, as well as in cells from STAT1 gain-of-function patients. These data indicate an interplay between STAT1 and STAT3 in fine-tuning IL-21 actions.