We report on the case of a 16-year-old boy with myocarditis developing after the third dose of COVID-19 mRNA vaccine. The patient presented with fever and retrosternal chest pain two days after immunisation. On admission, the boy was in good condition, with stable vitals. Elevated serum troponins and electrocardiographic repolarisation abnormalities with normal echocardiographic contractility were noted. Cardiac magnetic resonance imaging revealed active myocarditis in form of oedema with late gadolinium enhancement in five segments of the left ventricle myocardium. During further hospitalisation, rapid improvement in the patient’s general condition with normalisation of troponin level, and regression of electrocardiographic abnormalities were observed. In follow-up reassessments at three and nine months, cardiac magnetic resonance revealed residual myocardial lesions. No symptoms and abnormalities in laboratory tests, electrocardiography, or echocardiography were found. It is imperative to exercise full vigilance for COVID-19 vaccine-associated myocarditis when chest pain appears following vaccination, even in the absence of alarming symptoms after previous vaccine doses.