Background:Tenosynovial giant cell tumor (TGCT), formerly known as pigmented villonodular synovitis (PVNS), is a rare, benign but locally aggressive proliferative disorder of the synovial membrane. Its diffuse type is more aggressive than the localized type and carries a substantially higher risk of recurrence. Case presentation: We report the case of a 25-year-old man with no relevant medical history who presented with mechanical pain of the right knee evolving over 4 years. Clinical examination found a mildly swollen knee, tender on palpation and mobilization, with flexion limited to 110°, full extension, mild quadriceps amyotrophy, and normal ligamentous testing, without patellar tap sign or local inflammatory signs. Standard radiographs and inflammatory markers were normal. MRI showed diffuse-type TGCT involving both compartments of the knee, without extra-articular extension.