Cholecystocolonic fistula (CCF) is a rare biliary–enteric communication that often presents with nonspecific symptoms. We report the case of an 82-year-old woman who initially presented with large bowel obstruction caused by a 40-mm calcified intraluminal mass requiring sigmoid resection and colostomy. During her hospital stay, she developed acute cholecystitis with pneumobilia, and imaging studies suspected a CCF. Endoscopic ultrasound showed gallbladder wall thickening with intraluminal air and clear communication to the colon, confirming CCF. ERCP with biliary sphincterotomy was also performed. Colonoscopy revealed a 15-mm fistulous opening, which was treated with margin ablation using argon plasma coagulation and was closed with an over-the-scope clip. This case highlights the diagnostic challenges of CCF and supports endoscopic management as an effective alternative in high-surgical-risk patients.