Impingement syndrome developing on the lateral side of the patellofemoral joint (PFJ) resulting from soft tissues is extremely rare, and impingement syndrome resulting from an intra-articular synovial fold located in the inferolateral, parapatellar area has never been reported. We experienced a case in which a thickened, intra-articular synovial fold, which showed fibrous change and was located in the inferolateral parapatellar area in both knee joints, impinged on the PFJ, producing clinical symptoms. On physical examination, there was tenderness and a positive mediopatellar plica (MPP) test, which is a test of impingement, with no palpable cord-like painful thickening in the inferolateral parapatellar area of either knee joint. Magnetic resonance imaging showed entrapment of the intra-articular soft tissue in the PFJ in the inferolateral parapatellar area of both knee joints. On arthroscopy, thickened, whitish synovial tissue located in the inferolateral parapatellar area with fibrous change, which was well demarcated with an anterior fat pad, was seen causing impingement of the PFJ during knee extension. Histologically, there was no fatty tissue, just thick, fibrous tissue lined by synovial membrane. After its complete removal, the patient became asymptomatic.