AIM: The coronavirus disease 2019 (COVID-19) pandemic has left negative spillover effects on the entire health care system. Previous studies have suggested significant declines in cases of acute coronary syndrome (ACS) and primary percutaneous coronary intervention (PCI) during the COVID-19 pandemic. METHODS: We performed a quasi-experimental, retrospective cohort study of ACS hospitalisations by using a multi-institutional administrative claims database in Japan. We used interrupted time series analyses to ascertain impacts on cases, treatment approaches, and in-hospital mortality before and after Japan's state of emergency to respond to COVID-19. The primary outcome was the change in ACS cases per week. RESULTS: A total of 30,198 ACS cases (including 21,612 acute myocardial infarction and 8,586 unstable angina) were confirmed between 1st July 2018 and 30th June 2020. After the state of emergency, an immediate decrease was observed in ACS cases per week (-18.3%; 95% confidence interval, -13.1 to -23.5%). No significant differences were found in the severity of Killip classification (P=0.51) or cases of fibrinolytic therapy (P=0.74). The impact of the COVID-19 pandemic on in-hospital mortality in ACS patients was no longer observed after adjustment for clinical characteristics (adjusted odds ratio, 0.93; 95% confidence interval, 0.78 to 1.12; P=0.49). CONCLUSIONS: We demonstrated the characteristics and trends of ACS cases in a Japanese population by applying interrupted time series analyses. Our findings provide significant insights into the association between COVID-19 and decreases in ACS hospitalisations during the pandemic.