Malnutrition has been associated with adverse surgical outcomes, but data regarding its impact specifically in rectal cancer are sparse. The goal of this study was to use national data to determine the effects of malnutrition on surgical outcomes in rectal cancer resection. Data were obtained from the American College of Surgeons National Surgical Quality Initiative Program from the years 2012-2015. Patients were included on the basis of International Classification of Disease 9/10 and Current Procedural Terminology codes for rectal cancer and proctectomy. Malnutrition was defined as body mass index <18.5 kg/m2, weight loss >10% in 6 mo, or preoperative albumin <3.5 mg/dL. Associations between malnutrition and postoperative outcomes were assessed by the Student t-test and chi-square test. Multivariate regression models were constructed to adjust for potential confounders of the association between malnutrition and surgical outcomes. Of the 9289 patients with primary rectal cancer who underwent resection, 1425 (15%) were in a state of malnutrition at the time of surgery. Patients with malnutrition had longer mean length of stay (LOS), and higher rates of 30-d mortality, wound infection, organ-space infection, sepsis, reoperation, prolonged LOS (>30 d), failure to wean off ventilator, renal failure, and cardiac arrest. With the exception of LOS, renal failure, and organ-space infection, malnutrition was still significantly associated with higher rates of these adverse outcomes after adjustment for confounders in multivariate regression models. Malnutrition is a practical marker associated with a variety of adverse outcomes after rectal cancer surgery, and it represents a potential target for nutritional therapies to improve surgical outcomes.