A 81-year-old man with a history of gastrointestinal stromal tumor (GIST) was admitted to our hospital due to increased level of LDH, and further testing revealed a serum sIL-2R level of 15,489 U/ml. CT scan demonstrated retroperitoneal fibrosis without lymphadenopathy, and bilateral hydronephrosis. He died of aspiration pneumonia, and post-mortem biopsy revealed diffuse infiltration of medium to large-sized lymphoid cells positive for CD20, CD79a, and BCL2 in the retroperitoneal space with mild fibrosis. These findings suggested a diagnosis of primary retroperitoneal diffuse large B-cell lymphoma (PRLBCL). PRLBCL is extremely rare, and should be included in the differential diagnosis in cases of retroperitoneal fibrosis.