A 65-year-old man presented with a soft mass in his proximal right thigh. Ultrasonography showed a well-defined anechoic lesion with slightly internal echoes. On MRI, the mass was hypointense and minimally hyperintense compared with muscle at T1 and hyperintense at T2, with a hypointense peripheral rim on both sequences. No signal loss was observed on T1-weighted fat-suppression MRI. The clinical setting, imaging findings and histopathological features were consistent with a long-standing Morel-Lavallée lesion.