Mothers who have experienced childhood maltreatment are more likely to have children also exposed to maltreatment, a phenomenon known as intergenerational transmission. Factors in the perinatal period may contribute uniquely to this transmission, but timing effects have not been ascertained. Using structural equation modeling with 1,016 mothers and their 2,032 children in the Environmental Risk Longitudinal Twin Study, we tested the mediating role of postpartum depression between maternal childhood maltreatment and a cascade of negative child outcomes, specifically child exposure to maltreatment, internalizing symptoms, and externalizing symptoms: (a) adjusting for later maternal depression, (b) comparing across sex differences, and (c) examining the relative role of maltreatment subtypes. Mothers who had been maltreated as children, especially those who had experienced emotional or sexual abuse, were at increased risk for postpartum depression. In turn, postpartum depression predicted children’s exposure to maltreatment, followed by emotional and behavioral problems. Indirect effects from maternal childhood maltreatment to child outcomes were robust across child sex and supported significant mediation through postpartum depression; however, this appeared to be carried by mothers’ depression beyond the postpartum period. Identifying and treating postpartum depression, and preventing its recurrence, may help interrupt the intergenerational transmission of maltreatment and its sequelae.