Leiomyoma is a common benign tumor of smooth muscle cells that rarely occurs in the retroperitoneum. Here, we report a retroperitoneal leiomyoma with exophytic growing into the lesser sac incidentally diagnosed in a 69-year-old woman. In this case, the retroperitoneal leiomyoma presented as a well-circumscribed mass with heterogeneous signal intensity and mild progressive enhancement on magnetic resonance imaging. Differential image diagnosis of this retroperitoneal leiomyoma from other retroperitoneal tumors was difficult due to an overlap in imaging appearances with other retroperitoneal tumors.