Fibromyxoma is a rare benign odontogenic tumor of mesenchymal origin, but locally aggressive. Here, we report the case of a 47-year-old female who presented with a range of symptoms. These included a swollen left cheek, pain on the left side of her face, juicy discharge from the left nostril, and hearing loss in the left ear. The patient had been experiencing facial swelling on the left side for 6 months. On February 6, 2024, a biopsy of the paranasal cavity confirmed the presence of a fibromyxoma. The results of a crucial radiological examination revealed a large mass lesion suspected to be in the maxillary sinus, with expansion and mass effect and mild bone lysis. These findings played a pivotal role in determining the course of the patient's treatment. The lesion was totally excised, and the defect was filled with an anterolateral thigh (ALT) free flap. The histopathological examination confirmed the diagnosis of fibromyxoma. The patient's healing process was uneventful, reflecting the accuracy of the diagnosis. Complete tumor removal should be considered to ensure no remnants are attached to the soft tissue or bone due to the well-known potential for fibromyxomas to recur.