Primary rectal lymphoma is a rare presentation of gastrointestinal lymphomas. Its clinical presentation is indistinguishable from that of rectal carcinoma. Although surgical resection is often technically feasible, optimal therapy for colorectal lymphoma has not yet been identified. We report a case of primary rectal lymphoma (non-Hodgkin's large cell lymphoma of type B) with high-grade features that disappeared completely after chemo-radiotherapy. This case underlines that primary treatment with systemic chemotherapy and involved-field radiotherapy can be successful for rectal lymphoma, with surgery reserved for complications and chemotherapy failures.