Hereditary multiple osteochondromatosis (HMO) is characterized by multiple osteochondroma (OC) arising from enchondrally formed bones. Such lesions are seen most commonly in the forearm, knee and hip joint. In long bones, OC arises adjacent to the physis and tends to remain in the metaphyseal legion. Therefore, it is rare that OC is located intra-articular of the joint and only few cases of intra-articular OC have been previously reported. Intra-articular OC could appear as a single loose body in a joint cavity. Thus, other joint disorders, such as osteochondritis dissecans should be ruled out in such cases. We herein report a case with an intra-articular OC in a knee joint cavity in the patient with HMO.