The aim of this study was to evaluate the relationship between oral health literacy (OHL) among caregivers and untreated dental caries among preschoolers. A cross-sectional study was conducted involving 415 preschool children and their caregivers from Curitiba, Brazil. The children were examined for dental caries and dental plaque. The caregivers' OHL was evaluated using an instrument translated and validated for Brazilian Portuguese: the Brazilian Rapid Estimate of Adult Literacy in Dentistry-30. The prevalence of children with untreated caries was 42.9% (95%CI: 38.1-47.7%). A low degree of OHL was found among 15.7% (95%CI: 12.2-19.2) of caregivers. The multivariable Poisson regression analysis indicated a higher prevalence of untreated caries among children whose caregivers had a low OHL (PR = 1.35; 95% CI: 1.01-1.79), those with visible plaque (PR = 1.28; 95% CI: 1.01-1.62) and those who never or only occasionally had their teeth brushed by their caregivers (PR = 1.48; 95% CI: 1.16-1.91) in comparison to those without these conditions. In conclusion, a higher prevalence of untreated dental caries was found among preschool children whose caregivers had a low degree of OHL.