Leiomyosarcoma is an aggressive type of soft tissue neoplasm that originates from smooth muscle cells; it is typically found in the uterus, gastrointestinal tract, and retroperitoneum. Primary leiomyosarcoma of bone is an extremely rare phenomenon first reported in 1965. Here, we present a case of primary leiomyosarcoma of bone occurred in the proximal humerus of a 34–year–old man who was referred to our orthopaedic clinic with a chief complaint of right shoulder pain. Radiological investigations revealed a pathological fracture of right proximal humerus with a lytic lesion showing extensive periosteal reaction. Core needle biopsy of the lesion was performed, which pointed to primary leiomyosarcoma of bone as the underlying pathology.