Leydig cell tumour is a benign testicular non-germ cell tumour, and malignant transformation is rare. We report a case of a 35-year-old man who came to our hospital with a painless left testicular mass measuring 1.2 × 1 cm. Histological evaluation of the tumour showed features of a malignant Leydig cell tumour but no infiltration beyond the capsule or metastasis. The small size of the tumour was remarkable.