A 6-year old spayed female Siamese cat was presented with a history of left limb acute lameness. Radiographic imaging revealed a complete comminuted diaphyseal femur fracture, which was corrected through intramedullary pin placement along with plate osteosynthesis. After two months the animal returned to the hospital presenting severe diffuse enlargement of the left hindlimb, characterized as a non-delimited, solid, and firm plaque-like mass surrounding the pin and bone, which extended from the femorotibiopatelar joint to the pelvis, and infiltrated the adjacent musculature. Incisional biopsy exam indicated a highly malignant sarcoma. The entire limb was surgically removed and sent to histopathological evaluation. Microscopically, the mass was composed of spindle shaped cells, displaying high pleomorphism and cellular atypia. Abundant collagen production was evidenced through Masson's trichrome stain and strong cytoplasmic staining for vimentin. These results were consistent with fibrosarcoma. The cat went through one session of chemotherapy; however, tumor recurrence occurred 20 days later, and the animal was submitted to euthanasia. This is the first description of fibrosarcoma arising in the vicinity of an intramedullary pin and plate in a cat.