A 78-year-old woman presented with epigastric pain and underwent contrast-enhanced abdominal computed tomography (CT), which revealed a mass in the ampullary region. Endoscopic ultrasonography (EUS) and retrograde cholangiopancreatography (ERCP) confirmed a papillary lesion with a contrast defect in the bile duct. An initial biopsy suggested an adenoma. Owing to the extent of the lesion, surgery was recommended; however, the patient opted for follow-up. After five months, CT and EUS revealed significant tumor growth, raising the suspicion of malignancy. Consequently, subtotal stomach-preserving pancreatoduodenectomy was performed, and postoperative pathology confirmed an intra-ampullary papillary tubular neoplasm (IAPN) with low-grade dysplasia. IAPN is noninvasive and consists of the growth of the ductal epithelium, which is equivalent to adenoma or low-grade adenocarcinoma. Although it is generally characterized by slow growth owing to its noninvasive nature, this case was unique in that it showed rapid progression. Despite being a low-grade IAPN, this case represents a rare instance of rapid enlargement within five months. This case underscores the importance of vigilant monitoring of low-grade IAPN, given its potential for accelerated growth.