The claim that multiple partner fertility may pose a risk of adverse outcomes for children has not been tested. We test this argument using a sample of 4,027 resident fathers and children from the Fragile Families and Child Well-being Survey by examining the pathways through which fathers' multipartnered fertility is associated with children's externalizing behaviors and physical health status at 36 months. Path analyses indicate that multiple partner fertility exerted both a significant direct and indirect effect through paternal depression to influence children's externalizing behaviors. Fathers' multiple partner fertility also exerted a significant indirect effect through one mediator—father involvement—to influence children's physical health. This evidence suggests that the disruptions brought about by multipartnered fertility are important for understanding child well-being.