Soft tissue sarcomas are frequently reported tumours in dogs and are most often located in the cutaneous and subcutaneous tissues. Primary thoracic soft tissue sarcomas are infrequently reported in veterinary literature. This case report describes the diagnosis and treatment of a primary mediastinal spindle cell sarcoma of a 7‐year‐old, male, neutered Labrador retriever. Staging was performed before surgery. Computed tomography revealed a large intrathoracic mass in the left ventral hemithorax. A left‐sided thoracotomy was performed and a large pedunculated mass originating from the caudal mediastinum was excised. Histopathology revealed a high‐grade, undifferentiated spindle cell sarcoma. Immunohistochemistry staining confirmed the mesenchymal origin of the mass. Adjuvant chemotherapy was administered. Thoracic radiology was performed 38 days after surgery and revealed pleural effusion and mass effect in the caudal thorax. The patient was euthanased 66 days after surgery due to acute respiratory distress.