Introduction: Cap polyposis is a rare condition of the rectum or sigmoid colon manifested by inflammatory polyps covered by a thick layer of fibrinopurulent mucus. This condition typically presents as mucoid diarrhea and rectal bleeding, and patients are often prescribed antibiotics (such as those for Helicobacter pylori), steroids, infliximab, or aminosalicylates. Surgical management is an option for unresponsive disease, but endoscopic management has been rarely reported. Case Description/Methods: A 16-year-old boy had a 10-year history of mucoid diarrhea and occasional rectal bleeding and incontinence. Previous endoscopy revealed pseudopolyps in the rectum that appeared inflammatory with granulation tissue. Initial laboratory evaluation revealed mild iron deficiency, anemia, and hypoalbuminemia. The patient was treated with limited polypectomy, topical steroids, and diphenoxylate-atropine, which did not resolve his symptoms. A full workup for inflammatory bowel disease with an upper endoscopy, colonoscopy, and magnetic resonance enterography showed indications of marked polyposis in the rectum that was suggestive of "cap polyposis". Tests for H. pylori were negative. There was no improvement with metronidazole or with treatments to reduce straining during bowel movements. Repeat sigmoidoscopy showed diffuse 1-4-cm multilobulated polypoid lesions in the rectum. Initially ESD was considered but deemed unsuitable due to poor lifting of the lesions. Thus, wide field endoscopic mucosal resection (WEMR) was performed, Approximately 40 band ligations with snare endoscopic submucosal resection were performed with near complete removal of all of the lesions. There was no post-procedure bleeding, pain, or other adverse effects. The resected tissue specimen had signs of high-grade dysplasia away from the resection margins. The patient's symptoms resolved, and a second-look procedure 3.5 months later revealed only a slightly nodular area with no signs of dysplasia or cap polyposis. Discussion: Patients with cap polyposis typically report mucoid stools and rectal bleeding, and evaluations reveal a characteristic pattern on colonoscopy and pathology. For cases of cap polyposis in which conservative medical management fails, wide-field endoscopic mucosal resection is a viable option.