A 50-year-old woman presented with a 5-year history of mild pain in her right knee, which had increased over the last 2 years. A palpable mass over the anterolateral aspect of the knee was obvious and the last 3 months she was experiencing locking episodes with consequent knee effusion. The differential diagnosis was driven between meniscal cyst, pigmented villonodular synovitis, synovial sarcoma, synovial chondromatosis, and aneurysm. After a diagnostic arthroscopy, the lesion was excised by a limited lateral arthrotomy. The pathologic findings revealed a synovial cyst. Intra-articular synovial cysts are uncommon, nonsymptomatic, and mostly incidental findings on magnetic resonance imaging (MRI) and arthroscopy. This lateral meniscus synovial cyst (2.5 x 2.5 cm) was enlarged within the intracondylar notch and produced disabling knee symptoms. The peculiarity of this lesion was the tumor-like appearance: its large size, the progress of symptoms, and the multilobulated, nonhomogenous signal on the MRI scan. One year postoperatively, the patient is asymptomatic and the MRI obtained at 6 months revealed no remnant of the fully excised cyst.