Purpose It is unknown whether patients with advanced rectal cancer develop severe constipation. Therefore, the objective of this study was to assess whether constipation severity is associated with pathologic progression of rectal cancer. Methods We analyzed 472 patients with rectal cancer who underwent elective surgical resection between January 2005 and December 2010. Constipation severity was prospectively evaluated in 407 patients (86.2%) using the Cleveland Clinic Constipation Score System. Linear regression analysis was performed to identify clinicopathologic variables associated with constipation. Kaplan-Meier analysis and Cox proportional hazard models were used to evaluate the prognostic value of constipation severity on disease-free and overall survival. Results Multivariable analysis showed that sex (regression coefficient [B] = 1.55; 95% confidence interval [CI], 0.79 to 2.60; P < 0.001), body mass index (B = -0.95; 95% CI, -1.83 to -0.64; P = 0.036), tumor size (B = 1.04; 95% CI, 0.20 to 1.88; P = 0.016), T stage (B = 0.75; 95% CI, 0.23 to 1.27; P = 0.005), and distant metastasis (B = 1.16; 95% CI, 0.03 to 2.30; P = 0.045) were associated with constipation severity. Severe constipation (score ≥ 8) was independently associated with 3-year disease-free survival (vs. scores of 0-3; hazard ratio [HR], 2.39; 95% CI, 1.15 to 4.98; P = 0.020) and 5-year overall survival (HR, 2.30; 95% CI, 1.23 to 4.30; P = 0.009) in multivariable analysis. Conclusion Our results suggest that preoperative constipation severity is associated with advanced pathologic stage and poor oncologic outcomes in patients with rectal cancer.