A young, overweight dog presented with sudden onset lethargy and collapse following exercise in warm environmental conditions. Investigations revealed systolic hypotension, multiform ventricular premature complexes, irregular myocardial echogenicity with poor left ventricular systolic function and a markedly elevated troponin cTnI (180ng/mL, reference range <0.3ng/mL) consistent with severe myocyte damage. Infectious causes of myocarditis were ruled out on the basis of serological and polymerase chain reaction blood tests. Exercise-induced malignant hyperthermia was excluded from the history, an exercise tolerance test and genetic testing for the RYR1 V547A mutation. The diagnosis was myocardial damage secondary to suspected exertional heatstroke, from which the dog recovered uneventfully over a number of weeks and serum troponin normalised. This is the first case report in any species including man, documenting high troponin as a marker of severe myocardial damage following suspected heatstroke.