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We report a case of intracholecystic papillary neoplasm (ICPN) with associated invasive carcinoma mimicking adenomyomatosis. A woman in her 60s presented with epigastric and back pain. Ultrasonography showed gallbladder wall thickening with expanded Rokitansky-Aschoff sinuses (RAS), and diffusion-weighted MRI revealed no restricted diffusion. Adenomyomatosis was suspected, and laparoscopic cholecystectomy was performed. Histopathology revealed ICPN with invasive carcinoma. Tumour extension into the RAS with replacement of the luminal epithelium suggested occlusion of the RAS opening and secondary expansion. Additional resection showed no residual disease. No recurrence was observed during 2-year follow-up. This case highlights a diagnostic pitfall in which ICPN may mimic adenomyomatosis and suggests that irregular RAS expansion should be interpreted with caution.