A 37-year-old male with long-standing and extensive ulcerative pancolitis developed a rapidly lethal poorly differentiated neuroendocrine carcinoma (NEC) in the sigmoid colon. Prior biopsies obtained from multiple sites of the colon during endoscopic surveillance showed minimal inflammatory changes and no sign of dysplasia. Patients with inflammatory bowel disease (IBD) are at increased risk of colorectal malignancies, and adenocarcinoma is the most common type of colorectal neoplasm associated with ulcerative colitis and Crohn’s disease, but other types of epithelial and nonepithelial tumors have also been described in IBD. NECs arising in the setting of ulcerative colitis are very rare and are reported as anecdotic findings. We describe the clinicopathological features of an IBD-related NEC and review the previously reported cases.