Mesenteric cystic lymphangioma is an uncommon lesion that may be difficult to diagnose radiologically. In our case, a septate cystic mass showed fluid attenuation on a computed tomography scan. We were able to define the fat content within the cystic tumor by magnetic resonance (MR) imaging. The presence of fat within an intra-abdominal cystic mass is suggestive of a dermoid cyst, cystic lymphangioma, or lymphocele. The pathologic diagnosis revealed a cystic lymphangioma, which originated from the mesentery. Although the MR imaging features of dermoid cysts and lymphangiomas are well known, the demonstration of fat content by chemical shift and fat saturation MRI has not yet been reported for a cystic lymphangioma.