Maternal depression is a major risk factor for the development of children's mental health problems. No population-based study to date has examined whether early child care spanning the full preschool period from infancy onward is protective for children of depressed mothers. Objective: To examine whether early child care moderates associations between maternal depressive symptoms (MDSs) and child internalizing problems (emotional problems [EPs], separation anxiety symptoms, and social withdrawal symptoms [SWSs]) during the preschool period. Design and Setting: Population-based prospective cohort study within the Quebec Longitudinal Study of Child Development. Participants: Quebec Longitudinal Study of Child Development participants (n=1759) assessed repeatedly between ages 5 and 60 months. Main Outcomes and Measures: High-level trajectories of EPs, SWSs, and separation anxiety symptoms between ages 17 and 60 months. Results: Child care age at entry moderated associations between MDSs and child internalizing problems. Among children of mothers with elevated MDSs, reduced odds ratios for EPs and SWSs were found for those entering child care early (0.24; 95% CI, 0.09-0.66 for EPs and 0.29; 95% CI, 0.09-0.92 for SWSs) or late (0.29; 95% CI, 0.11-0.77 for EPs and 0.21; 95% CI, 0.07-0.65 for SWSs) compared with those remaining in maternal care. Child care type moderated the association between MDSs and child EPs; children of mothers with elevated MDSs who received group-based child care had lower odds ratios for EPs than those who remained in maternal care (0.21; 95% CI, 0.09-0.48) or those who were cared for by a relative or babysitter (0.40; 95% CI, 0.17-0.94). Conclusions and Relevance: Regulated early child care services reduced the risks for internalizing problems for children of mothers with elevated MDSs. Regulated child care services may be used as a public health intervention to buffer the negative effect of maternal depression on children's internalizing problems. (author abstract)