Background: The ToyBox-intervention has increased preschool children's water consumption. This study aimed to examine if family-related determinants mediate the effects of the ToyBox-intervention on preschoolers' water consumption. Methods: Overall, 6290 preschoolers and their families from 6 European countries participated in the ToyBox-intervention and returned parental questionnaires in May/June 2012 and 2013. This study included the 3725 preschoolers/families who had complete data on water consumption, all mediators, and confounders. Mediation effects were assessed with bootstrapping procedure. Results: Regarding the intervention effects on family-related determinants, the ToyBox-intervention significantly increased water availability during meals, parental water consumption, parental encouragement to their children to drink water, and parental knowledge on water recommendations. In the multiple mediator model, all factors were independently associated with preschoolers' water consumption and mediated the intervention effect on preschoolers' water consumption (total mediation effect = 40%). After including all mediators into the model, the direct intervention effect remained significant. Conclusions: The effect of the ToyBox-intervention on preschool children's water consumption was mediated by most family-related determinants examined in this study (ie, availability, parental modeling, parental encouragement, and parental knowledge). Interventions aiming to promote water in preschoolers should target these mediators to enhance their effectiveness.