We herein report the case of a 54-year-old Japanese man with hepatitis C virus (HCV)-related membranoproliferative glomerulonephritis (MPGN), which developed at the time of relapse of immune thrombocytopenic purpura (ITP) after rituximab therapy. Antiviral therapy for HCV led to the improvement of both MPGN and ITP. Rituximab therapy may have contributed to the exacerbation of HCV infection and induced the development of HCV-related MPGN and the relapse of ITP. Our case suggested that HCV treatment should be prioritized over rituximab therapy for HCV-positive patients with ITP and that antiviral therapy for HCV may be effective for treating ITP itself.