Purpose Malnutrition results in a significant increase in postoperative morbidity and mortality after abdominal surgery. Apart from the anthropometric assessments, malnutrition can be also assessed using laboratory scores, with the most widely used being Onodera's Prognostic Nutritional Index (PNI). The purpose of our study was to assess if the presence of malnutrition as calculated by the Onodera's PNI was associated with higher postoperative morbidity after elective colorectal cancer resection. Methods We performed a retrospective analysis of our institutional database including the patients who underwent elective colorectal cancer resection over a 24-month period. PNI scores were calculated and correlated amongst other parameters, such as cancer stage, severity of postoperative complications, unplanned transfusion of blood products, need for unplanned level 2/3 care after surgery and overall length of hospitalization. Results A total of 213 patients were included in this analysis, with 22.5% being classified as malnourished based on the preoperative PNI. Of note, PNI values were inversely associated with advanced-stage disease, severity of postoperative complications and unplanned intensive care unit (ICU) admission postoperatively. Also, malnourished patients had a statistically significant prolonged length of in-hospital stay. No difference in PNI scores was identified between groups requiring unplanned blood products' transfusions. Conclusions Preoperative malnutrition status as defined by PNI is associated with greater postoperative morbidity after elective surgery for colorectal cancer. Routine nutritional assessment and ad hoc nutritional support prior to surgery could contribute to an improvement of postoperative outcome after colorectal cancer resections.