The Prognostic Nutritional Index (PNI) is a valuable and informative indicator for clinicians. This study aims to investigate the impact of PNI on long-term survival in the geriatric population diagnosed with colorectal cancer. In this study, we collected demographic and clinicopathological data from 147 geriatric patients who underwent colorectal surgery at our institution between 2013 and 2022. PNI values were calculated for each patient. Based on ROC analysis for mortality, patients were categorized into two groups: low PNI and high PNI. With a median follow-up period of 34 months, the long-term outcomes of these groups were analyzed. The low PNI group (PNI ≤ 34.5) was associated with colon cancers rather than rectal cancer (p = 0.014), older age (p = 0.004), and higher mortality (p = 0.006). Additionally, in the survival analysis, the mean overall survival was 92.44 SD6.41 months in the high PNI group, compared to 76.66 SD8.77 months in the low PNI group (p = 0.006). For geriatric patients diagnosed with colorectal cancer who have a preoperative PNI ≤ 34.5, it is recommended that clinicians provide both preoperative and postoperative nutritional support, considering its significant impact on overall survival.