OBJECTIVE To investigate the clinicopathological features of testicular malignant Leydig cell tumor (TMLCT) and improve the non-invasive diagnosis of the disease. METHODS We retrospectively analyzed the clinicopathological data on a case of TMLCT, detected the circulating tumor cells (CTC) in the peripheral venous blood, and reviewed the related literature. RESULTS The patient, a 47-year-old male, underwent radical orchidoepididymectomy under general anesthesia. Postoperative pathology confirmed the lesion to be TMLCT, which was mainly composed of Leydig cells and suspected with vessel carcinoma embolus. Immunohistochemistry showed the tumor cells to be positive for α-inhibin, Ki67, CD30, vimentin, EMA, and PLAP, but negative for CK, CK7, S100, CD10, SMA, Des, AFP, hCG, CEA, CK19, CD117, Oct-4, LCA, CD20, Pax-5, CD3, and CD43. Two CTCs were detected in the peripheral venous blood. The patient received 3 courses of chemotherapy for retroperitoneal multiple lymph nodes metastasis post-operatively. Subsequent CT imaging manifested no obvious reduction of the retroperitoneal lymph nodes and consequently the patient again underwent retroperitoneal lymphadenectomy and cryoablation. At 8 months after treatment, CT examination revealed notably enlarged retroperitoneal lymph nodes with the right adrenal gland evidently invaded. CONCLUSIONS TMLCT is an extremely rare sex-gonad stromal tumor with high malignancy and poor prognosis, and CTCs may be used for its early diagnosis and prognostic prediction.