Objective Systemic inflammation and nutrition are associated with survival outcomes in metastatic colon cancer (mCC) patients. A new and strong prognostic marker named the Prognostic Immune Nutritional Index (PINI) was proposed as the best marker for outcomes in metastatic colon cancer patients. This study aimed to evaluate the prognostic significance of PINI in mCC patients. Methods The data of 190 patients who were admitted to our center and diagnosed with mCC between 2010 and 2020 abiding by our inclusion criteria were reviewed retrospectively. Receiver operating characteristic (ROC) analysis was used to identify the optimum cutoff value of PINI for overall survival (OS).Results The mean age of the participants was 62.64 +/- 11.99 years. The median follow-up time was 25.81 months. According to PINI, the median OS in patients who had PINI<3 was 22.70 months (95% confidence interval (CI): 16.05-29.35), and the median OS in patients who had PINI >= 3 was 38.83 months (95% CI: 26.98-37.01) (p<0.001). PINI score lower than 3 was an independent prognostic indicator in multivariate analysis.Conclusions PINI was discovered to be an independent prognostic factor in metastatic colorectal cancer. We believe that PINI, which can be calculated using a simple formula, will provide clinicians with important clues when deciding on individual treatment.