Background <p>This case report describes a large maxillary adenomatoid odontogenic tumor (AOT) initially misdiagnosed as a dentigerous cyst (DC) and highlights the postoperative use of a customized cyst plug to manage a palatal bone defect without bone grafting.</p> Case presentation <p>A 13-year-old girl presented with anterior maxillary swelling and a unilocular radiolucent lesion associated with an impacted canine. The lesion was initially diagnosed as a DC. Intraoperative findings prompted histopathologic examination, which confirmed AOT. A postoperative palatal bone defect was subsequently managed with a customized cyst plug. At 18-month follow-up, radiographic bone fill and satisfactory clinical healing were observed without additional grafting.</p> Conclusions <p>This case underscores both the diagnostic overlap between AOT and DC and the potential role of a customized cyst plug as a graft-sparing postoperative support in selected patients with large defects. Given the single-case design, therapeutic efficacy cannot be established, and further comparative studies are warranted.</p>