Cardiac glucose uptake and oxidation are reduced in diabetes despite hyperglycemia. Mitochondrial dysfunction contributes to heart failure in diabetes. It is unclear whether these changes are adaptive or maladaptive. To directly evaluate the relationship between glucose delivery and mitochondrial dysfunction in diabetic cardio-myopathy, we generated transgenic mice with inducible cardiomyocyte-specific expression of the GLUT4. We examined mice rendered hyperglycemic following low-dose streptozotocin prior to increasing cardiomyocyte glucose uptake by transgene induction. Enhanced myo-cardial glucose in nondiabetic mice decreased mito-chondrial ATP generation and was associated with echocardiographic evidence of diastolic dysfunction. Increasing myocardial glucose delivery after short-term diabetes onset exacerbated mitochondrial oxidative dys-function. Transcriptomic analysis revealed that the larg-est changes, driven by glucose and diabetes, were in genes involved in mitochondrial function. This glucose-dependent transcriptional repression was in part mediated by O-GlcNAcylation of the transcription factor Sp1. Increased glucose uptake induced direct O-GlcNAcylation of many electron transport chain subunits and other mitochondrial proteins. These findings identify mito-chondria as a major target of glucotoxicity. They also suggest that reduced glucose utilization in diabetic car-diomyopathy might defend against glucotoxicity and caution that restoring glucose delivery to the heart in the context of diabetes could accelerate mitochondrial dysfunction by disrupting protective metabolic adaptations.