The present invention provides binding proteins and antigen-binding fragments thereof that specifically bind to the human interleukin-21 receptor (IL-21R). The binding proteins can act as, e.g., antagonists of IL-21R activity, thereby modulating immune responses in general, and those mediated by IL-21R in particular. The disclosed compositions and methods may be used, e.g., in diagnosing and/or treating IL-21R-associated disorders, e.g., inflammatory disorders, autoimmune diseases, allergies, transplant rejection, cancer, and other immune system disorders.