This report describes the clinical manifestations and pathological findings in a 4-year-old neutered male dwarf rabbit diagnosed with a mandibular fibrosarcoma. The rabbit was evaluated for a unilateral mandibular swelling and an acute onset of oral blood loss. Physical examination confirmed a marked facial asymmetry with a swollen painful left mandible. Radiographic projections of the skull revealed a left mandibular mass extending into the oral cavity. Tissue biopsies demonstrated severe bone destruction and a presumptive pathological diagnosis of a mandibular osteosarcoma were made. A helical CT (computed tomography) study of the skull confirmed the severe mandibular bone destruction and the oral extension of the mass. The patient was treated palliative with oral meloxicam and subcutaneous penicillin injections. Six months later, the rabbit died in a natural way. Postmortem examination led to final diagnosis of severe left facial malformation due to mandibular fibrosarcoma. A bile duct adenoma was found in the liver. To the author’s knowledge, this is the first description of a spontaneous mandibular fibrosarcoma in a pet rabbit.