Background: Childhood obesity and dental caries are prevalent chronic, multifactorial conditions with adverse health consequences and considerable healthcare costs. The aims of this study were: (1) to evaluate the relationship between obesity and dental caries among young children using multiple definitions for both conditions, and (2) to evaluate the role of family socioeconomic status (SES) and the child’s intake of added sugars in explaining this association. Methods: Data from 2775 2−5-year-olds children from the National Health and Nutrition Examination Survey (NHANES) 2011−2018 were analysed. Three different international standards were used to define obesity, namely the World Health Organization (WHO), Centers for Disease Control and Prevention (CDC), and the International Obesity Task Force (IOTF). Dental caries was measured during clinical examinations and summarised as counts (dt and dft scores) and prevalence (untreated caries [dt > 0] and caries experience [dft > 0]). The association of obesity with dental caries was assessed in regression models controlling for demographic factors, family SES and child’s intake of added sugars. Results: In crude models, obesity was associated with greater dt scores when using the IOTF standards (RR: 2.43, 95% CI: 1.11, 5.29) but not when using the WHO and CDC standards; obesity was associated with greater dft scores when using the WHO (1.57, 95%CI: 1.11−2.22), CDC (1.70, 95%CI: 1.17−2.46) and IOTF standards (2.43, 95%CI: 1.73−3.42); obesity was associated with lifetime caries prevalence when using the WHO (1.55, 95%CI: 1.05−2.29), CDC (1.73, 95%CI: 1.14−2.62) and IOTF standards (2.45, 95%CI: 1.61−3.71), but not with untreated caries prevalence. These associations were fully attenuated after controlling for demographic factors, family SES and child’s intake of added sugars. Conclusions: The relationship between obesity and dental caries in primary teeth varied based on the definition of obesity and dental caries used. Associations were observed when obesity was defined using the IOTF standards and dental caries was defined using lifetime indicators. Associations were fully attenuated after adjusting for well-known determinants of both conditions.