Introduction: Parameniscal cysts are small cystic lesions, near the meniscus, involving medial and lateral compartments at equal frequency. Frequently, parameniscal cysts are so small that patients do not notice them, being asymptomatic. However, they can grow and exceed 2 centimeters in diameter, causing pain and alarm due to the slow growing mass. Magnetic Resonance Imaging (MRI) is the gold standard for diagnosis. Methods: Case report of a patient admitted to rheumatology department in the Centro Hospitalar e Universitário de Coimbra. Results: We report a case of a 47-year-old male with idiopathic juvenile arthritis, who presented with a slow-growing mass over the medial aspect of the right knee. MRI revealed a conspicuous cystic ovoid lesion, compatible with a parameniscal cyst, associated with structural heterogeneity of the posterior edge of the internal meniscus with a longitudinal fracture at this level. Conclusion: This is the first case of parameniscal cyst reported in patients with inflammatory rheumatic disease and the differential diagnosis with synovial cyst, baker cyst, ganglion cyst, bursitis, hematoma and neoplasms is of utmost importance.