<p>This study aimed to evaluate the nonlinear association between prognostic nutritional index (PNI) and mortality in geriatric hip fractures. Geriatric patients with hip fractures were screened between January 2015 and September 2019. We collected the demographic information and followed up on the survival data. Multivariate Cox proportional hazards regression model and restricted cubic spline were used to identify the linear and nonlinear associations between PNI and post-operative mortality. This study included two thousand one hundred and fifteen patients who met the criteria. There were 1443 females, accounting for 68.23% of all patients. The mean age was 79.36 ± 6.74 years. The mean follow-up period was 38.57 ± 17.96 months. There were 636 all-cause deaths. In primary linear association, PNI was associated with mortality (HR = 0.95; 95%CI: 0.93–0.97; P &lt; 0.0001) in the adjusted model. However, it was nonlinear that the real association between PNI and mortality. Through two-piecewise Cox proportional hazards regression model analysis, we found that 50.3 was an inflection point. When PNI was less than 50.3, there was an association between PNI and mortality (HR = 0.94; 95%CI: 0.92–0.96; P &lt; 0.0001). For every one-unit increase in PNI, the mortality of patients decreased by 6%. When PNI was greater than 50.3, the mortality became relatively stable and no longer changed with PNI (HR = 1.03, 95%CI:0.99–1.07; P = 0.1469). The results were stable when the patients were divided into subgroups as different confounding factors in stratification analysis. PNI is a vital predictor of mortality for patients with hip fractures. We found that 50.3 was a significant clinical inflection point. When the PNI is less than 50.3, the mortality decreases as the PNI increases. When PNI is more than 50.3, the mortality rate tends to be stable and maintained at a low level. Therefore, PNI should be assessed when the elderly hip fractures for preoperative risk stratification and beyond its threshold. </p>