Individuals with diabetes mellitus frequently develop severe skin and soft tissue infections (SSTIs) that are recalcitrant to antibiotic treatment. We examined how diabetes affects the emergence of antibiotic resistance in a Staphylococcus aureus SSTI. We determined that S. aureus evolves antibiotic resistance rapidly in diabetic mice, while resistance did not occur in nondiabetic mice over the course of infection. Diabetes-associated immune cell dysfunction plays a minor role in the emergence of resistance, while hyperglycemia plays a dominant role facilitating the expansion and takeover of resistant mutants in diabetic infections. Furthermore, vancomycin intermediate resistant isolates display a pronounced fitness defect in nondiabetic mice but not in diabetic mice. Together, these data suggest that the diabetic infection environment represents an ideal reservoir for the emergence and proliferation of antibiotic resistance. Controlling the blood sugar of diabetic mice with insulin resulted in significantly decreased incidence of antibiotic-resistant S. aureus.