This study investigated the association between parental long working hours and the depression of children living with their parents even after becoming adults in South Korea. Data from two years of the Korean National Health and Nutrition Examination Survey (KNHANES) in 2014 and 2016 were analyzed using a cross-sectional study. Logistic regression analyses were performed to determine the association between parental long working hours and their adult children's depression. Adult children of mothers who worked more than 60 hours per week were more likely to have depression than 40 hours or less per week. Comprehending aging mothers' weekly working hours is an important factor for understanding the reason of adult children's depression.