Diabetes exacerbates ischemic heart disease morbidity and mortality via incompletely understood mechanisms. Although adiponectin (APN) reduces myocardial ischemia/reperfusion (MI/R) injury in nondiabetic animals, whether APN's cardioprotective actions are altered in diabetes, a pathologic condition with endogenously reduced APN, has never been investigated. High-fat diet (HD)-induced diabetic mice and normal diet (ND) controls were subjected to MI via coronary artery ligation, and given vehicle or APN globular domain (gAPN, 2 μg/g) 10 min before reperfusion. Compared to ND mice (where gAPN exerted pronounced cardioprotection), HD mice manifested greater MI/R injury, and a tripled gAPN dose was requisite to achieve cardioprotective extent seen in ND mice (i.e., infarct size, apoptosis, and cardiac function). APN reduces MI/R injury via AMP-activated protein kinase (AMPK)-dependent metabolic regulation and AMPK-independent antioxidative/antinitrative pathways. Compared to ND, HD mice manifested significantly blunted gAPN-induced AMPK activation, basally and after MI/R (p<0.05). Although both low- and high-dose gAPN equally attenuated MI/R-induced oxidative stress (i.e., NADPH oxidase expression and superoxide production) and nitrative stress (i.e., inducible nitric oxide synthase expression, nitric oxide production, and peroxynitrite formation) in ND mice, only high-dose gAPN efficaciously did so in HD mice. We demonstrate for the first time that HD-induced diabetes diminished both AMPK-dependent and AMPK-independent APN cardioprotection, suggesting an unreported diabetic heart APN resistance.