Background Limited data on the relationship between postoperative complications (POCs) after colorectal cancer resection and oncologic outcomes are available. We hypothesized that the increased severity of POCs is associated with progressively worse oncologic outcomes. Methods Patients with pathological stages I–III colorectal adenocarcinoma undergoing elective curative resection in a single institution between 2000 and 2012 were identified from a prospectively collected database. The severity of POCs was determined using the Clavien–Dindo classification, and oncologic outcomes were assessed. Results Of 2266 patients, 669 (30%) had at least one POC. POCs were not associated with pathologic stage ( p  = 0.58) or use of adjuvant therapy ( p  = 0.19). With a mean follow-up of 5.3 years, POCs were associated with decreased 5-year overall survival (OS) (60% vs. 77%, p  < 0.001), disease-free survival (DFS) (53% vs. 70%, p  < 0.001), cancer-specific survival (CSS) (81% vs. 87%, p  < 0.001), and increased overall recurrence rates (19% vs. 15%, p  = 0.008). Increasing Clavien–Dindo scores from I to IV was significantly associated with progressively decreasing OS (71, 64, 60, 22%, p  < 0.001), DFS (65, 58, 51, 19%, p  < 0.001), CSS (88, 77, 79, 74%, p  < 0.001), and increasing recurrence rates (12, 20, 26, 18%, p  = 0.002). Multivariate analysis confirmed POCs as an independent factor associated with decreased OS hazard ratio (HR) 0.63, 95% CI 0.52–0.76, DFS (HR 0.64, 95% CI 0.54–0.76), CSS (HR 0.73, 95% CI 0.56–0.97), and increased recurrence rates (HR 1.36, 95% CI 1.02–1.80). Conclusions POCs are associated with adverse oncologic outcomes, with increasing effect with higher Clavien–Dindo score. Efforts to reduce both the incidence and severity of complications should result in improved oncologic outcomes.