Young ethnic minority parents may lack psychological and financial resources to handle parenthood, increasing the risk of negative psychosocial and parenting outcomes. Partner support has been associated with positive coparenting, although findings have been mixed. Support from young parents’ own parents (“grandparents”) has been linked to adaptive family outcomes and may be particularly protective for African American and Latino parents whose cultures espouse interdependence. This study examined partner support and grandparent support as individual predictors of change in coparenting quality, and tested whether grandparent support moderated the relationship between partner support and change in coparenting quality over the first postpartum year. Participants were 136 African American and Latina adolescent mothers (age range = 15–21 years) and their babies’ fathers (15–41 years). Partner and grandparent support were measured at 6 months postpartum. Coparenting quality was measured at 6 and 12 months postpartum, and change in coparenting quality was measured using latent change scores. Structural equation modeling was used to test the hypothesis that the relationship between partner support and change in coparenting quality would be moderated by grandparent support. Fit indices indicated a well-fitted model. Results demonstrated that the moderator term (partner support × grandparent support) significantly predicted change in coparenting quality. Specifically, partner support was positively associated with changes in coparenting quality when grandparent support was high; however, that association became weaker and changed direction for lower levels of grandparent support. Findings highlight the need to assess parents’ social support networks and grandparents’ impact on the coparenting quality of this at-risk population.