Introduction: Chronic knee pain in young adults can often be challenging to diagnose due to its wide array of potential underlying conditions. This case report delves into a particularly rare and intricate case involving a18-year-old male who presented with giant cell tumor of the tendon sheath (GCTTS) within the knee joint, an atypical occurrence for this benign lesion. Typically, GCTTS originates from the synovium of joints, bursae, or tendon sheaths, with a higher prevalence in the fingers. However, its manifestation in the knee is exceptionally rare, making this case unique in terms of patient age and tumor size. Case presentation: The patient's initial presentation consisted of non-specific symptoms, including infra-patellar pain and the development of a growing swelling, posing a diagnostic challenge. Employing magnetic resonance imaging (MRI) played a pivotal role in the identification of a well-circumscribed soft tissue mass. The diagnosis of GCTTS was established through histopathological examination following surgical excision. Conclusion: This case report aims to raise awareness of the potential occurrence of GCTTS in young patients with knee pain and non-specific symptoms. Early and accurate diagnosis is underscored as essential for appropriate treatment and management, especially given the rarity and diagnostic intricacies and broad differential diagnoses associated with this condition. Favorable prognosis can be achieved following successful surgical excision, when GCTTS is promptly diagnosed and treated in atypical locations, such as the knee joint, demonstrating the significance of heightening awareness among healthcare providers and clinicians to consider GCTTS in patients with knee pain and swelling.