Mucosa-associated lymphoid tissue (MALT) lymphoma is a typical primary gastrointestinal lymphoma, particularly in the stomach. Although primary rectal lymphoma is rare, it may present as a subepithelial tumor. Several techniques have been proposed for a tissue diagnosis in subepithelial tumor, including endoscopic ultrasonography (EUS)-guided fine needle aspiration (EUS-FNA), EUS-guided trucut biopsy (EUS-TCB), and tacked biopsy. However the diagnostic efficacy of these techniques appears to be limited. The unroofing technique involves removal of the overlying mucosa, thereby exposing the subepithelial lesion. It was originally reported as a method for endoscopic treatment of colorectal lymphangioma. In this case, a subepithelial tumor of the rectum was diagnosed using the endoscopic unroofing technique. This is a useful modality for the diagnosis of subepithelial tumor, because it provides histologic results in a safe and rapid manner.