A 43-year-old man, a regular drinker, developed a pseudocyst in the pancreatic tail as a result of acutely worsening chronic pancreatitis. Because the pseudocyst, 10 cm in diameter, did not disappear despite conservative treatment, an internal drainage stent was placed transgastrically under endoscopic ultrasound (EUS) guidance. However, cyst infection occurred, and EUS-guided drainage was performed, when the gallbladder was punctured inadvertently. Immediately a nasocystic drain was placed in the gallbladder. Owing to this timely measure, only mild and localized peritonitis developed. Conservative treatment with fasting and an antibiotic was administered, and peritonitis subsided quickly. On the same day, another nasocystic drain was placed for the pancreatic pseudocyst, and it disappeared. As far as we know, this is the first case in which gallbladder puncture was inadvertently performed during EUS-guided drainage of a pancreatic pseudocyst.