Abstract Background & Aims: The long-term risk of colorectal neoplasia in ulcerative colitis (UC) patients undergoing colectomy remains poorly quantified. This study compared the incidence of colorectal malignancies in UC patients with versus without colectomy across 5-year, 10-year, and 20-year follow-up. Methods: Using the TriNetX US Collaborative Network, we conducted a propensity score-matched cohort study of 3,767 UC patients with colectomy (partial/total) and 3,767 non-colectomy UC controls, matched for age, sex, race, and family cancer history. Outcomes included incident colon (C18), rectosigmoid (C19), and rectal (C20) cancers analyzed at 5-year, 10-year, and 20-year intervals. Risk ratios (RR), hazard ratios (HR), and survival probabilities were calculated. Results: Colectomy patients had significantly higher risks of colorectal cancers. Colon cancer incidence was 20.0% at 5 years (vs. 1.6%, HR 13.2), persisting through 10 and 20 years. Rectosigmoid cancer risk peaked at 5 years (HR 20.4), remaining elevated at 20 years (HR 14.4). Rectal cancer risk was consistently increased (HR range 7.4–8.3). 20-year survival was significantly lower in colectomy patients. Conclusions: Colectomy for UC is associated with a markedly increased risk of colorectal malignancy, particularly in the first 5 years for colon cancer and persistently for rectosigmoid and rectal cancers. Lifelong surveillance is warranted post-colectomy.