Mucosa-associated lymphoid tissue (MALT) lymphoma predominantly involves the gastrointestinal tract, with the stomach being the most commonly affected site. Colonic involvement is quite rare. Hence, the etiology, clinical characteristics, treatment, and outcome of colonic MALT lymphoma are not well established. This paper reports a case of MALT lymphoma of the transverse colon, presenting as a subepithelial tumor in a 50-year-old woman. The patient received 3,060 cGy in 17 fractions with external beam radiation therapy for three weeks. At 2 months after radiation therapy, a colonoscopy examination revealed complete resolution and a scar change of the lesion. The follow-up at 24 months revealed no evidence of tumor recurrence after radiation therapy.