This study presents a case report of an inflammatory dentigerous cyst of tooth #35, associated with its previously endodontically treated deciduous predecessor. Cystic lesion growth caused impaction of the second premolar, displacing it closer to the lower border of the mandible. The lesion represents a typical dentigerous cyst that possibly arises in association with periapical inflammation of a deciduous molar involving the follicle of the premolars. This report highlights the inflammatory etiology of dentigerous cysts, which mainly occur in mixed dentition. A 12-year-old patient was referred to Oral Surgery Department regarding a sizeable radiolucent lesion in the unerupted mandibular second premolar region, detected on an Orthopantomagram (OPG) X-ray. A non-vital primary predecessor had been endodontically treated at least one year before an examination, with a control OPG X-ray showing no signs of pathology at the time. The patient did not report any symptoms. Clinical examination revealed an egg-like tumefaction of the alveolar bone in the left premolar region of the mandible. Cone-beam computed tomography analysis showed a sizeable translucent lesion surrounding the crown of the impacted tooth. The lesion was enucleated in its entirety, along with the impacted premolar, under local anesthesia. Clinical findings combined with radiographic and microscopic examinations confirmed the diagnosis of an inflammatory dentigerous cyst. The seventeen month follow-up revealed good bone healing. This case presented a rare complication of endodontic treatment of deciduous teeth and informed on possible complications of endodontic therapy in deciduous teeth, emphasizing the importance of early diagnosis of cysts in preventing extraction of permanent teeth.