ObjectiveDuring the coronavirus disease 2019 (COVID-19) pandemic, informal caregivers’ mental health deteriorated more than that of non-caregivers. We examined the association between increased caregiver burden during the pandemic and severe psychological distress (SPD). MethodsWe used cross-sectional data from a nationwide internet survey conducted between August and September 2020 in Japan. Of 25,482 participants aged 15–79 years, 1,920 informal caregivers were included. SPD was defined as Kessler 6 Scale (K6) score ≥ 13. Self-rated change in caregiver burden was measured retrospectively with a single question item. Binary logistic regression analysis was used to examine the association between SPD and increased caregiver burden during the pandemic, adjusted for demographic, socioeconomic, health, and caregiving variables. To examine the differential association between increased caregiver burden and SPD, interaction terms were added and binary logistic regression was separately conducted for all variables. ResultsParticipants’ mean age was 52.3 years (standard deviation 15.9), 48.8% of participants were male, 56.7% reported increased caregiver burden, and 19.3% exhibited SPD. Increased caregiver burden was significantly associated with SPD (adjusted odds ratio: 1.90; 95% confidence interval: 1.37–2.66). The association between increased caregiver burden and SPD was stronger among caregivers who were married, those undergoing disease treatment, and those with a care-receiver with a care need level of 1–2. ConclusionsThe results revealed that more than half of caregivers reported increased caregiver burden, and increased caregiver burden was associated with SPD during the pandemic. Measures supporting mental health for caregivers with increased caregiver burden should be implemented immediately.