Purpose: Cancer patients receiving various anti-cancer treatments commonly experience malnutrition, and many studies have reported that nutritional status is associated with survival and prognosis. Although standard neoadjuvant chemoradiotherapy (CRT) is commonly used in patients with locally advanced rectal cancer owing to its tumor-downsizing and downstaging effects, there is a lack of research on the impact of patients’ nutritional status on the efficacy of neoadjuvant CRT. Methods: We investigated the immunonutritional markers before and after long-course neoadjuvant CRT in 131 patients diagnosed with locally advanced rectal cancer from March 2013 to March 2022. Results: We divided the patients into two groups: a low prognostic nutritional index (PNI) with a cutoff value of 50.92, and a high PNI. In both groups, significant decreases in lymphocyte count and PNI and an increase in neutrophil-to-lymphocyte ratio (NLR) were observed before and after CRT (P<0.001). Furthermore, a higher proportion of patients experienced adverse effects in the low PNI group than in the high PNI group (76.6% in low PNI vs. 54.8% in high PNI, P=0.013). The most commonly reported CRT-induced adverse effect was lower gastrointestinal tract toxicity. Conclusion By measuring the PNI and NLR without additional tests prior to starting neoadjuvant CRT in patients with locally advanced rectal cancer, it is possible to predict the risk of acute adverse effects caused by CRT. Additionally, providing external nutritional support to reduce the immunonutritional changes that occur during CRT can decrease side effects and potentially increase treatment compliance.