We find substantial and statistically significant detrimental effects of fathers' multiple-partner fertility (MPF) on children's educational outcomes. We focus on children in fathers’ “second families” when the second families are nuclear families – households consisting of a man, a woman, their joint children, and no other children. We analyze outcomes for almost 75,000 Norwegian children, all of whom, until they were at least age 18, lived in nuclear families. Controlling for a rich set of socioeconomic variables, we find that children who spent their entire childhoods in nuclear families but whose fathers had children from a previous relationship living elsewhere were 4 percentage points more likely to drop out of secondary school and 5 percentage points less likely to obtain a bachelor's degree than children in nuclear families without fathers’ MPF. Resource competition due to economic and caregiving responsibilities for children living elsewhere does not explain the differences in educational outcomes. We do find that children in nuclear families whose fathers had previous childless marriages have educational outcomes that are similar to those of fathers with MPF. Our analysis suggests that the effects of fathers' MPF are primarily due to selection.