Lipoma arborescens is a rare intra-articular lesion of unknown etiology. We present a 70-year-old woman with a 10-year history of recurrent swelling and effusion in the right knee. Histological examination after surgical synovectomy confirmed the diagnosis of lipoma arborescens. The fatty nature and frond-like appearance of the tumor was demonstrated on magnetic resonance imaging (MRI). Open synovectomy is the curative treatment. Although this syndrome is rare, it is important to recognize it and make differential diagnosis from other synovial lesions using MRI.