Pharmacological preconditioning reduces myocardial infarct size in ischaemia‐reperfusion (I‐R) injury. Dexmedetomidine, a selective α2‐adrenoceptor agonist, has a proven cardioprotective effect when administered prior to I‐R, although the underlying mechanisms for this effect are not fully understood. We evaluated whether dexmedetomidine preconditioning could induce a myocardio‐protective effect against I‐R injury by inhibiting associated inflammatory processes through downregulation of the high mobility group box 1 (HMGB1)‐toll‐like receptor 4 (TLR4)‐nuclear factor κB (NF‐κB) signalling pathway. Seventy rats were randomly assigned to seven groups: a control and six test groups, involving I‐R for 30 and 120 minutes, respectively, in isolated rat hearts and different pretreatment protocols with dexmedetomidine (10 nmol/L) as well as yohimbine (1 μmol/L) and recombinant HMGB1 peptide (rHMGB1; 20 μg/L), alone or in combination with dexmedetomidine. Cardiac function was recorded; myocardial HMGB1, TLR4, and NF‐κB activities and levels of tumour necrosis factor‐α (TNF‐α) and interleukin‐6 (IL‐6) were measured as were lactate dehydrogenase (LDH) and creatine kinase (CK) in coronary outflow. Dexmedetomidine preconditioning significantly improved cardiac function (P<.05), downregulated the expression of HMGB1‐TLR4‐NF‐κB, reduced levels of TNF‐α and IL‐6 in isolated ventricles during I‐R injury, and significantly reduced CK and LDH levels in coronary outflow (P<.05). All of these effects were partially reversed by yohimbine (P<.05) or rHMGB1 (P<.05). Dexmedetomidine preconditioning alleviated myocardial I‐R injury in rats through inhibition of inflammatory processes associated with downregulation of the HMGB1‐TLR4‐NF‐κB signalling pathway via activation at α2‐adrenergic receptors.