Introduction: Non-traumatic knee joint effusion and fullness is a relatively common presenting complaint among athletes and nonathletes. Due to its broad differential diagnosis, a comprehensive evaluation beginning with history and physical examination are recommended. Imaging including plain radiography, magnetic resonance imaging (MRI), and in some cases ultrasound are preferred modalities. If inflammatory arthritis is suspected, joint aspiration and analysis may help diagnosis. Case Presentation: A 37-year-old male soccer player presented with a complaint of left anterior knee pain and fullness for a few months. Physical examination revealed a healthy appearing male with obvious fullness of his left suprapatellar pouch and posterolateral knee. Plain radiographs were unremarkable. MRI demonstrated an effusion infiltrated by multiple, low intensity projections from a fatty mass in the suprapatellar pouch consistent with lipoma arborescens. Conclusions: Lipoma arborescens is a rare synovial disorder characterized by replacement of subsynovial tissue with mature fat cells, most commonly in the knee joint. MRI is the best diagnostic modality to evaluate and confirm the diagnosis as well as rule out other pathologies. More recent single case-reports and clinical series endorse arthroscopic synovectomy as the treatment of choice.