Under the COVID-19 outbreak, the Japanese government has strongly encouraged individuals to stay at home. The aim of the current study was to clarify the effects of the COVID-19 outbreak on the lifestyle of older adults with dementia or mild cognitive impairment (MCI) who live alone. Seventy-four patients with dementia or MCI aged ≥65 years, who regularly visited the dementia clinic of the Department of Psychiatry, Osaka University Hospital, were recruited in this study. The patients were divided into two groups according to their living situation: living alone group ( = 12) and living together group ( = 62). Additionally, the spouses of patients aged ≥65 years were assigned to the healthy control group ( = 37). Subjects' lifestyle changes were evaluated between April 8 and 28, 2020. No subjects with acquaintances or relatives were infected with COVID-19 within the study period. The proportion of subjects who reduced going out in the living alone group, living together group and healthy control group was 18.2, 52.5, and 78.4%, respectively. The proportion of subjects who went out less frequently was significantly lower in both the living alone ( < 0.01) and living together ( < 0.05) groups than in the healthy control group. Most patients with dementia or MCI who live alone did not limit their outings or activities during the COVID-19 outbreak. Regular monitoring for potential COVID-19 infection in people living alone with dementia is vital for their safety and well-being.