Lipoma arborescens, also known as villous lipomatous transformation of synovium is an unusual fatty lesion of joint. This disorder is characterized by a diffuse increase in the quantity of subsynovial fat in supra-patellar region which bulges the overlying synovial lining and produces a villous architecture. Hereby we report a case of Lipoma arborescens in an eleven year old boy with a history of supra-patellar swelling of left knee since a year ago. Also, he had a vague history of trauma before left knee swelling. Radiological and histopathologic features of our case are discussed alongside full review of literature. This disorder is an unusual lesion and occurs mainly in middle aged men and is rare in pediatric age group.