Diagnostic esophagogastroduodenoscopy (EGD) is generally a very safe procedure. We report the first case of iatrogenic esophageal submucosal dissection after an attempted diagnostic gastroscopy in a patient with a small previously undiagnosed Zenker's diverticulum (ZD). After EGD, she developed severe dysphagia with the inability to swallow solids, liquids, and even her own saliva. On barium swallow study, there was a column of contrast below the upper esophageal sphincter, and this was misdiagnosed as a large ZD by the radiologist. The resultant stricture was successfully managed with endoscopic balloon dilatation under fluoroscopy with wire-guided cannulation. The ZD was treated with flexible endoscopic clip-assisted diverticulotomy. Iatrogenic submucosal dissection is a unique complication of upper endoscopy. Endoscopists, otolaryngologists, radiologists, and cardiothoracic surgeons should be aware of this condition and prepare to manage it appropriately. If the patient is stable and the possibility of perforation is small, conservative and supportive care can be tried first. A surgical gastrostomy tube can be placed for enteral feeding. In patients with ZD, ZD recognition and gentle manipulation is strongly recommended during esophageal intubation.