Sarcomas commonly occur in the buttock and thigh, although the clinical presentation varies with no specific symptoms other than a painless lump. This case study reports on a soft-tissue sarcoma that was initially thought to be a haematoma on ultrasound, despite being rescanned 6 weeks later. The patient presented back to their general practitioner 8 months later with the subsequent ultrasound showing an irregular and hypervascular soft-tissue mass. Further imaging and ultrasound-guided biopsy led to a surgical excision of a myxofibrosarcoma. This case demonstrates the difficulty of differentiating between a haematoma and sarcoma, especially in patients presenting with a history of trauma.