Endoscopic resection is one of the curative options for early gastric cancer. However, based on large-scale data about the risk of lymph node metastasis in early gastric cancer, endoscopic resection has been attempted for the following: differentiated intramucosal gastric cancer, regardless of size and without ulcers; differentiated intramucosal cancer, 30 mm in size with ulcers; minute submucosal differentiated cancer <30 mm in size; and undifferentiated intramucosal cancer, <20 mm in diameter without venous or lymphatic involvement. However, undifferentiated early gastric cancer exhibits different biologic behavior from differentiated early gastric cancer. Thus, the application of endoscopic resection for undifferentiated early gastric cancer remains controversial. In this review, we discuss the application of endoscopic resection for undifferentiated early gastric cancer based on analysis of biologic behavior and data of endoscopic resection.