This retrospective cross-sectional study aimed to evaluate the predictive value of SII (Systemic Immune Inflammation Index) and PNI (Prognostic Nutritional Index) with blood ratios on mortality in diabetic foot patients who underwent below-knee amputation. A total of 231 living (n = 71; 30.7%) and exitus (n = 160; 69.3%) patients were evaluated. The mortality group was divided into 3 groups: 30-day mortality (n = 62; 38.8%), 1-year mortality (n = 62; 38.8%), and over-1-year mortality (n = 36; 22.5%). The hemogram, SII, and PNI parameters of the patients were evaluated. Age, some blood count parameters and SII were significantly higher in the exitus group (P <.05). The lymphocyte, monocyte, eosinophil, albumin, and PNI levels were significantly higher in the living group (P <.05). Mortality was significantly predicted by age (B [regression coefficient] = 0.026, P <.05), NLR (neutrophil lymphocyte ratio) (B =-0.065, P <.05), PNI (B =-0.100, P <.01), and SII (B = 0.00000024, P <.01). The predictive values of CAR (C reactive protein albumin ratio), PNI, and SII were 77.3%, 77.0%, and 76.1%, respectively. For CAR of 30.88 cutoff value, the sensitivity and specificity were 79.4% and 64.8%, respectively. For the PNI 22.0143 cutoff value, the sensitivity and specificity were 66.9% and 5.6%, respectively. For the SII 732249.2481 cutoff value, the sensitivity and specificity were 91.9% and 31.0%, respectively. The predictive value of the PNI was significant for mortality time (B = 0.058; P <.01). The predictive value of PNI for 30-day mortality was significant (AUC (area under curve):0.632; P <.01), whereas its predictive value for 1-year mortality and over-1-year mortality after below-knee amputation was statistically insignificant (P >.05). Both the SII and PNI may be evaluated and used to predict mortality after below-knee amputation. The SII had a significant predictive value for 30-day mortality after below-knee amputation.