The coronavirus disease 2019 (COVID-19), first identified in December 2019, has significantly affected healthcare workers' mental health (HCWs). The authors have provided psychosocial support to HCWs working in typical hospitals and nursing homes in Fukushima Prefecture, Japan, where major COVID-19 outbreaks have occurred since December 2020. This study examines depressive symptoms among HCWs working at typical hospitals and nursing homes experiencing in-house major COVID-19 outbreaks in a retrospective manner. We have offered psychosocial support in eight hospitals and nursing homes, obtaining data on the mental health status of 558 HCWs. The study’s results indicate that 29.7% of HCWs have exhibited moderate or higher depressive symptoms, and 10.2% had suicidal ideation. Multiple regression analysis shows that being a nurse and nursing home staff is associated with higher depressive symptoms. In addition, multiple regression analysis of PCR (Polymerase Chain Reaction)-positive HCWs shows that being a nurse and the number of COVID-19-related symptoms are associated with high depressive symptoms. These results suggest that HCWs in typical hospitals and nursing homes experiencing major COVID-19 outbreaks are more likely to exhibit severe depressive symptoms, such as suicidal ideation, which may worsen if infected with COVID-19. This study’s findings expand the current understanding of HCWs’ depressive symptoms and the importance of psychosocial support during unexpected major outbreaks in healthcare facilities.