Colorectal surgery is associated with a high incidence of postoperative complications regardless of the advances in surgical techniques and multidisciplinary treatment. Proteinuria is common in patients with malignancies, but few studies have investigated the association between preoperative proteinuria and patient prognoses, especially postoperative complications. We investigated the impact of proteinuria on patients undergoing colorectal surgery in a single-center, retrospective cohort study of 767 patients who underwent surgical resection for colorectal cancer between January 2016 and December 2022 at the National Hospital Organization Shikoku Cancer Center. Among them, 81 patients with preoperative proteinuria were compared with the control group of 686 patients without proteinuria. Our analyses revealed that the patients with proteinuria had malnutrition with a significantly lower prognostic nutritional index compared to the no-proteinuria control group (p<0.001). The proteinuria group had a significantly advanced tumor stage (p=0.005), experienced more bleeding during the surgery (p=0.002), and required more transfusions (p<0.001). Postoperative complications were significantly more frequent in the proteinuria group (p=0.03), thus demonstrating that proteinuria was independently associated with postoperative complications (p=0.045). Proteinuria in patients undergoing colorectal cancer surgery can therefore be considered a risk factor for postoperative complications.