Rituximab treatment may cause or exacerbate Kaposi's sarcoma (KS) in patients with human immunodeficiency virus (HIV)-associated multicentric Castleman's disease. Despite the widespread use of rituximab, rituximab-induced KS has not yet been reported in HIV-negative patients with diffuse large B cell lymphoma (DLBCL). We herein report a case of KS that developed after undergoing rituximab-containing chemotherapy in an HIV-negative patient with DLBCL. An 84-year-old man who received rituximab-containing chemotherapy for the treatment of DLBCL developed severe infection, and subsequently KS. Our observations indicate that serious infections under rituximab treatment may trigger KS. KS should therefore be considered when skin tumors appear in lymphoma patients receiving rituximab-containing chemotherapy.