Theories of family functioning suggest that childbearing with multiple partners may increase parenting stress due to changes in social and economic resources and the challenges associated with parenting across multiple households. These family processes may not be equally stressful for mothers and fathers, because they face different parental constraints and responsibilities. I use four waves of data from the Fragile Families and Child Wellbeing Study to examine whether multi-partnered fertility increases parenting stress for mothers and fathers. Using lagged regression models and longitudinal repeated reports of parenting stress, I find that both mothers and fathers report increases in parenting stress following the birth of a child with a new romantic partner, relative to parents who experience no additional childbearing. However, increases in parenting stress following multi-partnered fertility are similar to increases in parenting stress following same-partner fertility. I also find that transitions to new romantic partnerships are associated with increases in parenting stress for mothers and fathers only when new romantic partners have children from previous relationships. Increases in parenting stress following all fertility transitions are stronger for less-educated parents than for more-educated parents.