A 74-year-old man underwent regular follow-up observations after being diagnosed with gallbladder adenomyomatosis based on findings, such as the thickening of the wall of the gallbladder fundus and the presence of intramural cysts. Over the course of 3 years, a papillary tumor located on the thickened wall of the gallbladder had increased in size and extended into the lumen. Consequently, the patient was diagnosed with gallbladder cancer and underwent extended cholecystectomy. The histological diagnosis was intracystic papillary neoplasm (ICPN) of the gallbladder. Although several Rokitansky-Aschoff sinuses that had increased in size because of tumor progression were observed, no adenomyomatosis of the gallbladder was detected. ICPN, a recently identified disease, is not widely known to present with imaging findings similar to adenomyomatosis. The primary treatment of ICPN is radical resection, whereas adenomyomatosis is generally conservatively managed with regular follow-up observations. As the treatment strategies for these two diseases greatly differ, differential diagnosis must be carefully performed.