A 34-year-old woman visited our clinic with complaints of trismus, numbness in the left half of the tongue, and pain in tooth 34. At the patient’s initial visit to our clinic, the maximum assisted jaw opening movement was 20 mm, pointing to severe trismus. The patient complained of spontaneous pain in tooth 34 but did not evince percussion pain. Her pain remained unchanged even under local anesthesia. Radiography showed no pathological findings. A tenderness test of masticatory muscles failed to induce pain. The presence of severe trismus argued against temporomandibular disorders, resulting in suspicion of other inflammatory disease. In view of numbness of the tongue, other diseases such as inflammation or neoplastic disease in the head and neck region were considered. The patient was referred to the departments of neurosurgery and otolaryngology for examination. The results of MRI and CT diagnosis led to the identification of acute myositis of the left medial pterygoid muscle requiring the prescription of nonsteroidal anti-inflammatory drugs, and relaxation of mandibular muscles. With time, maximum unassisted jaw opening improved and pain in tooth 34 ceased, but tongue numbness persisted. The condition was attributed to compression of lingual nerve (LN) and mandibular nerve (MN) resulting from medial pterygoid muscle inflammation.