Introduction: Adenoid cystic carcinoma (ACC) is one of the most common salivary gland malignancies, mostly occurs in the major and minor salivary glands in the oral and maxillofacial region. The development of ACC in the retromolar pad is extremely rare, which limits establishing proper diagnosis and management. Presentation of case: A patient described a 2-month history of finding a mass behind the lower left posterior teeth. Based on the physical examination and radiographic findings, we got an initial impression of a benign mucocele, the nature of which was to be investigated further. Pathological examination of the resected tissue resulted in a diagnosis of ACC. Follow-up visits showed no recurrence during the subsequent 54 months. Discussion: In cases with an uncertain diagnosis based on medical history, clinical features and imaging examinations, it is important to proceed carefully with the possibility of a tumor in mind. Conclusion: ACC in the retromolar pad is rare and can be easily misdiagnosed. Clinical, radiographic, and pathological evidence confirm a definitive diagnosis. Long-term follow-up is important for the full analysis of ACC treatment.