Primary mucinous adenocarcinoma of the appendix is a rare gastrointestinal malignancy. Fistulous tract formation is a complication that is cited in literature. An 85-year-old man with multiple comorbidities presented with several weeks of persistent non-bloody diarrhea. Laboratory work-up was non-diagnostic. Abdominal imaging with barium contrast showed an enterocolonic fistulous tract extending from the duodenum to the cecum involving an enlarged appendiceal mass. Subsequent biopsy confirmed mucinous appendiceal neoplasm with peritoneal spread to the liver and mesentery. This is the first report describing an enterocolonic fistula formation resulting from this malignancy.