A 74-year-old woman with a chief complaint of hematochezia was admitted to our hospital. Colonoscopy revealed a 2 cm submucosal tumor with an erosion in the Rb of the rectum. Biopsy showed various inflammatory cell clusters, but no malignant findings. Thoracoabdominal CT and abdominal MRI showed no abnormal findings other than that of rectal wall thickening. Somatostatin receptor scintigraphy performed on suspected rectal NET showed no abnormal accumulation. A total of 3 biopsies were performed, but a definitive diagnosis was not confirmed, and transanal rectal mucosal resection was performed for diagnostic purposes. Immunohistochemistry showed CD20(+)and bcl-2(+), and the lesion was diagnosed as malignant rectal B cell lymphoma. For 1 year postoperatively, the patient showed no recurrence. For this case, biopsy often failed to confirm a definitive diagnosis on rectal submucosal tumors. If the tumor is >1 cm in diameter in a rectal NET, a rectal resection with lymph node dissection is required, and anal function may be impaired. Local tumor excision for diagnosis and treatment may be worthwhile in rectal submucosal tumors if pre-resection diagnosis with biopsy is difficult.