Tehran University of Medical Sciences Journal of Orthopedic and Spine Trauma 2538-2330 9 4 2023 12 30 192 4 Seyyed-Morteza Kazemi Professor, Bone Joint and Related Tissues Research Center, Akhtar Orthopedic Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran Reza Minaie-Noshani Assistant Professor, Bone Joint and Related Tissues Research Center, Akhtar Orthopedic Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran Seyyed-Mohsen Hosseininejad Bone Joint and Related Tissues Research Center, Akhtar Orthopedic Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran; Joint, Bone, Connective Tissue Rheumatology Research Center (JBCRC), Golestan University of Medical Sciences, Gorgan, Iran 2023 01 17 2023 12 30 Background: Tenosynovial giant cell tumor (TGCT) is a rare hyperplastic disease of the synovial membrane of the joint with a high recurrence rate and tumor-like features, affecting large joints such as the hip, knee, and ankle. Dull and chronic pain is common, as is swelling of the joint, which can make definitive diagnosis difficult. The gold standard of treatment is complete resection of the lesion. In this case report, we present a TGCT at the knee joint of an adult patient with nonspecific clinical presentation. Case Report: We present a 33-year-old male patient who presented with chronic swelling, warm sensation, and limited range of motion (ROM) in the knee. He had no history of trauma and infectious disease had been ruled out. After several nonspecific orthopedic procedures and inadequate treatment of signs and symptoms, he was finally diagnosed with local TGCT. Conclusion: To make the correct diagnosis, the unusual and somehow deceptive clinical presentation of TGCT must be considered. https://jost.tums.ac.ir/index.php/jost/article/view/472 https://jost.tums.ac.ir/index.php/jost/article/download/472/343