Systemic lupus erythematosus (lupus) is characterized by autoantibody-mediated organ injury. Follicular Th (Tfh) cells orchestrate physiological germinal center (GC) B cell responses, whereas in lupus they promote aberrant GC responses with autoreactive memory B cell development and plasma cell–derived autoantibody production. IL-21, a Tfh cell–derived cytokine, provides instructional cues for GC B cell maturation, with disruption of IL-21 signaling representing a potential therapeutic strategy for autoantibody-driven diseases such as systemic lupus erythematosus. We used blockade of IL-21 to dissect the mechanisms by which this cytokine promotes autoimmunity in murine lupus. Treatment of lupus-prone B6. Sle1. Yaa mice with an anti–IL-21 blocking Ab reduced titers of autoantibodies, delayed progression of glomerulonephritis and diminished renal-infiltrating Tfh and Th1 cells, and improved overall survival. Therapy inhibited excessive accumulation of Tfh cells coexpressing IL-21 and IFN-γ, and suppressed their production of the latter cytokine, albeit while not affecting their frequency. Anti–IL-21 treatment also led to a reduction in GC B cells, CD138 hi plasmablasts, IFN-γ–dependent IgG2c production, and autoantibodies, indicating that Tfh cell–derived IL-21 is critical for pathological B cell cues in lupus. Normalization of GC responses was, in part, caused by uncoupling of Tfh–B cell interactions, as evidenced by reduced expression of CD40L on Tfh cells and reduced B cell proliferation in treated mice. Our work provides mechanistic insight into the contribution of IL-21 to the pathogenesis of murine lupus, while revealing the importance of T–B cellular cross-talk in mediating autoimmunity, demonstrating that its interruption impacts both cell types leading to disease amelioration.