The present application provides human antibodies and antigen binding fragments thereof that specifically bind to the human interleukin-21 receptor (IL-21R). The antibodies can act as 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 for example, in diagnosing, treating or preventing inflammatory disorders, autoimmune diseases, allergies, transplant rejection, cancer, and other immune system disorders.