Globally many children are living with grandparents, and it has been suggested that grandparent co-residence may be associated with dental caries in infants and toddlers possibly through passive parenting style, accompanied by children's cariogenic behaviors such as feeding sugary sweets. However, little is known about this association in schoolchildren, adjusted for socioeconomic status. Therefore, this study investigates the association between grandparent co-residence, socioeconomic status, and dental caries among schoolchildren. All caregivers of first-grade children (age 6-7 years) in Adachi City, Tokyo, were administered a questionnaire about children's grandparent co-residence status and oral health-related behaviors, and responses were linked with dental examination records conducted by school dentists (N = 3,578). Multilevel Poisson regression analysis was applied to examine the association between grandparent co-residence, socioeconomic status, and dental caries status for each individual tooth, adjusting for potential covariates. The percentage of dental caries experience was higher among children living with grandparents (48.9%) than among children living without grandparents (44.0%). The risk for caries, however, did not differ according to grandparent co-residence status when tooth type, child's age and sex, and parental socio-economic status and structure were adjusted (PR, 1.13; 95%CI, 0.90, 1.42). The association between grandparent co-residence and dental caries among early school-aged children in urban Japan was confounded by socioeconomic status.