BACKGROUND Hemosiderotic synovitis (HS) is a rare proliferative synovial disorder with incompletely understood pathophysiology. It mainly affects the knee joint. It can be confused with pigmented villonodular synovitis (PVNS), both clinically and radiologically. HS has not been previously reported in children, and this rare incidence may lead to difficulties in early clinical and radiological diagnosis, possibly affecting the patient's outcome. CASE REPORT A 13-year-old boy presented with progressive right knee pain, swelling, and limitation of movement 2 months after a traumatic injury in a soccer game. His past medical and family history was unremarkable. His physical exam showed right knee effusion, fullness in the popliteal fossa, and painful restriction in active and passive knee motion. Laboratory tests showed a mildly elevated erythrocyte sedimentation rate. Knee X-rays showed joint effusion. Knee MRI showed large knee joint effusion with diffuse low signal intensity villous synovial hypertrophy in all sequences in addition to a large popliteal cyst with the same imaging characteristics, consistent with diffuse-form PVNS. Total arthroscopic synovectomy was performed without complications. Intraoperatively, there was a large effusion containing yellow clots. The histopathological diagnosis was post-traumatic HS with no evidence of PVNS or malignancy. The patient showed significant clinical and radiological improvement postoperatively with no evidence of recurrence. CONCLUSIONS Post-traumatic HS is a very rare entity that should always be considered in the differential diagnosis of synovitis in active, non-hemophilic children. Radiologists' familiarity with this rare condition is important to guide correct and early diagnosis, and to avoid unnecessary invasive intervention.