Background: Temporomandibular disorder (TMD) is a broad term that encompasses pain and/or dysfunction of the masticatory musculature and TM joints (TMJs). Its most important feature is pain, followed by limited jaw movement, and joint sounds. When it progresses to a chronic condition, the symptoms are extremely difficult to manage, often requiring multiple interventions. Case presentation: Our patient, a woman in her 50s, developed TMD after a traffic accident that occurred 30 years previously. The patient presented with severe trismus due to TMJ pain (maximum mouth opening was 20 mm). She was scheduled to undergo extraction of a right lower molar and three upper anterior teeth owing to progressing caries. However, it was anticipated that the treatment would be difficult since the patient could not open her mouth adequately. Therefore, we considered relieving the trismus to facilitate dental treatment. Based on the findings, the cause of the TMD was considered to be pain in the masticatory muscles during mouth opening. Ultrasound-guided inferior alveolar nerve block (IANB) was performed on both sides using ropivacaine. Three minutes after the IANB, the pain during mouth opening disappeared and the maximum mouth opening improved to 40 mm. Dental treatment could be performed without difficulty, and the patient could maintain the mouth open throughout the treatment.Conclusions: Treatment methods for chronic TMD are limited and it is necessary to consider the exact aetiology before deciding upon a treatment option. In our case, ultrasound-guided IANB proved to be an effective modality for relieving TMD-related trismus.