Objective To investigate the relation of neutrophil percentage to albumin ratio (NPAR) with all⁃cause mortality risk in patients with diabetes and cancer comorbidities based on National Health and Nutrition Examination Survey (NHANES). Methods Based on the NHANES data of 1258 patients with diabetes and cancer comorbidities included from 1999 to 2018 in the NHANES database, the receiver operating characteristic (ROC) curve was adopted to determine the optimal cut⁃off value of NPAR in predicting all⁃cause mortality in patients with diabetes and cancer comorbidities. Patients were assigned to high NPAR group (664 cases) or low NPAR group (594 cases) based on the optimal cut⁃off value. The survival curves of the two groups were drawn by the Kaplan⁃Meier method, and the survival curves of the two groups were compared by the log⁃rank test. The multivariate COX regression model was adopted to analyze the relation between NPAR and all⁃cause mortality risk, and the subgroup analysis was conducted. The dose⁃response relation of NPAR with all⁃cause mortality risk was analyzed by employing restricted cubic spline (RCS) and threshold effects. Results The median follow⁃up time of 1258 patients with diabetes and cancer comorbidities was 89 months, and 512 patients died, with a mortality rate of 40.7%. The ROC curve analysis revealed that the optimal cut⁃off value of NPAR in predicting all⁃cause mortality in patients with diabetes and cancer comorbidities was 14.639. The survival curve analysis suggested that all⁃cause mortality risk in the low NPAR group was lower than that in the high NPAR group (P<0.05). The results of multivariate COX regression model expressed that all⁃cause mortality risk in the high NPAR group was 1.60 times higher than that in the low NPAR group after adjusting for confounding factors (P<0.05). The RCS analysis and threshold effect analysis interpreted that a U⁃shaped nonlinear relation between NPAR and all⁃cause mortality risk (Pnonlinear<0.05). When NPAR≤12.819, all⁃cause mortality risk of patients with diabetes and cancer comorbidities decreased with the increase of NPAR, whereas when NPAR>12.819, all⁃cause mortality risk of patients with diabetes and cancer comorbidities increased with the increase of NPAR (P<0.05). Subgroup analysis exhibited that there was no interaction effect of the correlation of NPAR with all⁃cause mortality risk in patients with diabetes and cancer comorbidities between various subgroups (Pinteraction effect>0.05). Conclusion There is a U⁃shaped non⁃linear relation between NPAR and all⁃cause mortality risk in patients with diabetes and cancer comorbidities. NPAR can be used as an independent predictor of all⁃cause mortality risk in patients with diabetes and cancer comorbidities.