OBJECTIVE Limited studies have examined the association between diabetes and HbA1c with postoperative outcomes. We investigated the association of diabetes, defined categorically, and the association of HbA1c as a continuous measure, with postoperative outcomes. RESEARCH DESIGN AND METHODS In this prospective, observational study, we measured the HbA1c of surgical inpatients age ≥54 years at a tertiary hospital between May 2013 and January 2016. Patients were diagnosed with diabetes if they had preexisting diabetes or an HbA1c ≥6.5% (48 mmol/mol) or with prediabetes if they had an HbA1c between 5.7 and 6.4% (39 and 48 mmol/mol). Patients with an HbA1c <5.7% (39 mmol/mol) were categorized as having normoglycemia. Baseline demographic and clinical data were obtained from hospital records, and patients were followed for 6 months. Random-effects logistic and negative binomial regression models were used for analysis, treating surgical units as random effects. We undertook classification and regression tree (CART) analysis to design a 6-month mortality risk model. RESULTS Of 7,565 inpatients, 30% had diabetes, and 37% had prediabetes. After adjusting for age, Charlson comorbidity index (excluding diabetes and age), estimated glomerular filtration rate, and length of surgery, diabetes was associated with increased 6-month mortality (adjusted odds ratio [aOR] 1.29 [95% CI 1.05–1.58]; P = 0.014), major complications (1.32 [1.14–1.52]; P < 0.001), intensive care unit (ICU) admission (1.50 [1.28–1.75]; P < 0.001), mechanical ventilation (1.67 [1.32–2.10]; P < 0.001), and hospital length of stay (LOS) (adjusted incidence rate ratio [aIRR] 1.08 [95% CI 1.04–1.12]; P < 0.001). Each percentage increase in HbA1c was associated with increased major complications (aOR 1.07 [1.01–1.14]; P = 0.030), ICU admission (aOR 1.14 [1.07–1.21]; P < 0.001), and hospital LOS (aIRR 1.05 [1.03–1.06]; P < 0.001). CART analysis confirmed a higher risk of 6-month mortality with diabetes in conjunction with other risk factors. CONCLUSIONS Almost one-third of surgical inpatients age ≥54 years had diabetes. Diabetes and higher HbA1c were independently associated with a higher risk of adverse outcomes after surgery.