Background: Preoperative diagnosis of lymph node metastasis from colorectal cancer is crucial; guidance for its diagnosis based on radiological findings is lacking.Aims: In this study, we validated the diagnostic accuracy of preoperative computed tomography for lymph node metastasis from colorectal cancer.Methods: Patients who underwent surgery for colorectal cancer from January 2015 to August 2020 and from January 2007 to December 2014 were enrolled in the derivation (n = 250) and validation cohorts (n = 141), respectively. Computed tomography was performed using a contrast agent and 5-mm-thick sections. The characteristics of the largest lymph nodes detected through computed tomography were estimated.Results: In the derivation cohort, receiver operating curve analysis revealed optimal cutoff values of 7.4 and 4.9 mm for the long-axis and short-axis diameters, respectively, based on the large area under the curve values (0.76 and 0.78, respectively). The sensitivity, specificity, and accuracy at a cutoff of 7 mm for the long-axis diameter were 65%, 78%, and 72%, respectively. In the validation cohort, univariate and multivariate analyses revealed that a long-axis diameter ≥ 7 mm was a significant predictor of lymph node metastasis.Conclusions: A lymph node long-axis diameter ≥ 7 mm is a predictive factor of lymph node metastasis in patients with colorectal cancer.