Background: Synovial sarcoma is an intermediate- to high-grade malignant soft tissue tumor that accounts for approximately 2.5–10% of all soft tissue sarcomas. It most commonly affects adolescents and young adults (15–40 years), with a slight male predominance. Despite its name, it does not show true synovial differentiation and often arises in periarticular soft tissues, especially around the knee. Clinically, synovial sarcoma presents as a slowly growing mass, which may be mistaken for a benign lesion, especially in the absence of significant pain. Radiologic features may include a well-circumscribed mass with peripheral calcifications (seen in up to 30% of cases), with MRI being the preferred imaging modality due to its superior soft tissue contrast. Findings such as the “triple sign” on T2-weighted MRI and early gadolinium enhancement may support the diagnosis. The presented case highlights the diagnostic challenges: a 38-year-old woman experienced knee discomfort for 18 months, initially misinterpreted as a benign cystic lesion on ultrasound. Definitive diagnosis was achieved only after MRI and surgical biopsy confirmed synovial sarcoma. This underscores the importance of early recognition and imaging in patients with persistent, unexplained soft tissue masses. Key words: Synovial sarcoma, soft tissue neoplasm, malignant tumor, periarticular mass and popliteal fossa.