ABSTRACT In endoscopic full‐thickness resection (EFTR), luminal collapse can occur due to communication between the gastrointestinal lumen and peritoneal cavity, making visualization and procedural continuity difficult. We propose the underwater pressure lumen expansion (UPLE) method, in which hydraulic pressure is applied in a fluid‐filled environment to re‐expand the collapsed lumen, thereby restoring visualization and allowing continued endoscopic manipulation. The UPLE method was applied during peroral endoscopic tumor resection in a clinical case and in an animal model of EFTR, and its utility was successfully demonstrated.