Talon cusp is an odontogenic anomaly in anterior teeth, caused by hyperactivity of enamel in morphodifferentiation stage. Talon cusp is an additional cusp with several types based on its extension and shape. It has enamel, dentin, and sometimes pulp tissue.  Moreover, it can cause clinical problems such as poor aesthetic, dental caries, attrition, occlusal interferences, and periodontal diseases. Therefore, early diagnosis and effective treatment of talon cusp are essential. Maxillary incisors are the most commonly affected teeth. However, occurrence of mandibular talon cusp is a rare entity. We report a talon cusp in the lingual surface of the permanent mandibular left central incisor, in a 7-year-old Iranian boy. To our knowledge it is the third case reported in Iranian patients. Also in this case, molar-incisor hypomineralization  in permanent mandibular first molars and permanent incisors as well as delayed eruption of permanent maxillary left central incisor were observed. So far, there is no report about talon cusp with this type of developmental defect.