Severe infection, including sepsis, is an increasing clinical problem that causes prolonged morbidity and substantial mortality. At present, antibiotics are essentially the only pharmacological treatment for sepsis. The incidence of resistance to antibiotics is increasing; therefore, it is critical to find new therapies for sepsis. Staphylococcus aureus is a major cause of septic mortality. Neutrophils play an important role in the defense against bacterial infections. We have shown that a diet with high levels of dietary saturated fatty acids decreases survival in septic mice, but the mechanisms behind this remain elusive. The aim of the present study was to investigate how the differences in dietary fat composition affect survival and bacterial load after experimental septic infection and neutrophil function in uninfected mice. We found that, after S. aureus infection, mice fed a polyunsaturated high-fat diet (HFD-P) for 8 weeks had increased survival and decreased bacterial load during sepsis compared with mice fed a saturated high-fat diet (HFD-S), similar to mice fed a low-fat diet (LFD). Uninfected mice fed HFD-P had a higher frequency of neutrophils in bone marrow than mice fed HFD-S. In addition, mice fed HFD-P had a higher frequency of neutrophils recruited to the site of inflammation in response to peritoneal injection of thioglycolate than mice fed HFD-S. Differences between the proportion of dietary protein and carbohydrate did not affect septic survival at all. In conclusion, polyunsaturated dietary fat increased both survival and efficiency of bacterial clearance during septic S. aureus infection. Moreover, this diet increased the frequency and chemotaxis of neutrophils, key components of the immune response to S. aureus infections.