Peroral endoscopic myotomy (POEM) is an entirely endoscopic approach for the treatment of achalasia. This new procedure has been shown to be safe, effective, and associated with only minor complications in the postoperative period. This case report describes the development of atrial fibrillation after POEM secondary to direct compression from a hematoma in the submucosal tunnel. To our knowledge, this is the first report of a delayed hematoma after POEM. This procedure is still novel, and it is important to continue to share information about potential complications and long-term results. This report also includes several interesting radiographic images to illustrate what occurred. Finally, we provide a brief review of the literature on complications that have been described after POEM.