Socioeconomic inequalities contribute to childhood overweight. Identifying mediators could help reduce these inequalities. We assessed to what extent and how parental health literacy and health behaviours mediate the relationship between parental socioeconomic status and childhood overweight. Data were taken from the multigenerational prospective Dutch Lifelines Cohort Study. We included 6683 children, baseline age 9.8 years (SD = 2.6), with an average follow-up of 36.2 months (SD = 9.3). Overweight was defined using age- and sex-specific cut-offs. Three indicators of socioeconomic status were included: education, income and occupation. We assessed the mediating role of parental health literacy and health behaviours (smoking, diet, physical activity and alcohol) using causal mediation. Four additional years of education and an SD-increase in both income and occupation decreased the odds of childhood overweight by 42%, 12% and 20%, respectively. Only parental smoking independently mediated the relationship of both education (6.6%) and occupation (5.7%) with overweight. Parental health behaviours jointly explained 8.4% (education), 19.4% (income) and 9.8% (occupation) per relationship. Lastly, adding parental health literacy explained 10.8% (education), 27.4% (income) and 13.3% (occupation) of these relationships. We found large socioeconomic inequalities in childhood overweight. Remarkably, parental smoking was a key mediator. Therefore, prevention targeting smoking may reduce socioeconomic inequalities in childhood overweight.