Following myocardial infarction (MI), patients with type 2 diabetes mellitus (T2DM) have poorer prognosis which may be linked to increased susceptibility of coronary microvessels to injury. Interleukin-36 (IL-36) may mediate this injury but its role in the microcirculation of the chronically hyperglycaemic injured heart is unknown. Intravital and laser speckle imaging of the anaesthetised mouse beating heart evaluated the impact of a 16-week high fat diet (HFD)-induced hyperglycaemia ± myocardial ischaemia-reperfusion injury (IR) injury on coronary microvessels. Neutrophil/platelet recruitment, neutrophil extracellular trap formation, cellular necrosis, vascular leakage, vascular tonal changes, functional capillary density, overall ventricular perfusion and levels of circulating inflammatory cytokines were assessed alongside the vasculoprotective ability of an IL-36 receptor antagonist (IL-36Ra). Whether heightened microvessel damage in injured HFD mice was permanent or reversible was investigated after normalising hyperglycaemia through diet reversal (DR). Microcirculatory events assessed were perturbed basally in HFD mice and further after injury. IL-36Ra mitigated these effects and improved infarct size. DR was also beneficial, decreasing neutrophil recruitment to levels below those seen in untreated mice. Mechanistically, benefits of both IL-36Ra and DR could be explained by decreased endothelial oxidative stress and VCAM-1 expression and possibly by raised levels of IL-4/IL-13. Basal changes in chronically hyperglycaemic coronary microvessels that are heightened in the aftermath of reperfusion may explain the poorer outcomes in MI patients with T2DM. These findings are the first to highlight the specific benefits of IL-36 inhibition and reversing hyperglycaemia through dietary modification on the coronary microcirculation in a preclinical model of T2DM.