Castleman disease represents a rare lymphoproliferative disorder of unknown etiology. It is usually located in the mediastinum and in very few cases in the retroperitoneal space. We present the case of a 43-year-old male patient with a retroperitoneal tumor that was incidentally diagnosed during an abdominal computed tomography scan. The patient underwent surgery by open approach, and the tumor, which was adherent to the superior pole of the left kidney, was entirely removed. The histology examination revealed a vascular-hyaline-type Castleman disease. The postoperative evolution was uneventful, with no signs of tumor recurrence at the 4-month check-up. The surgeon should be aware of the possible retroperitoneal location of Castleman disease, even if it is a rare occurrence, and a complete removal of the tumor is followed by a favorable long-term prognosis.