Canine parvovirus (CPV) is a highly contagious disease, typically causing haemorrhagic enteritis in dogs. The myocardial form, though well-documented historically, is now rare in vaccinated canine populations. This case report describes the first confirmed outbreak of acute parvoviral myocarditis in one-day-old German shepherd puppies in Iran. A litter of six neonates from a reportedly vaccinated primiparous bitch presented with acute lethargy, refusal to nurse, hypothermia and respiratory distress. Despite supportive medical care, all puppies died within hours. Postmortem examination revealed cardiomegaly with pale myocardial streaks, pulmonary consolidation and thymic haemorrhage. Histopathological analysis of heart tissue identified lymphoplasmacytic myocarditis with intranuclear inclusion bodies. Polymerase chain reaction (PCR) performed on cardiac tissue confirmed the presence of CPV. This case underscores that the cardiac form of CPV remains a threat to neonates, likely due to in utero or perinatal infection in the absence of maternal immunity. This case also serves as a reminder that CPV myocarditis can have a delayed clinical course, with heart failure and death occurring weeks to months after initial infection. Although CPV myocarditis can have a delayed clinical course, with heart failure and death occurring weeks to months after initial infection, this case underscores that the cardiac form of CPV remains a threat to neonates, likely due to in utero or perinatal infection in the absence of maternal immunity. It highlights the critical importance of ensuring complete and timely vaccination of breeding bitches and the need for vigilance regarding this rare but fatal manifestation of CPV.