Non-marital childbearing in the US has reached historic levels. Because of the instability of nonmarital partnerships, multipartnered fertility, whereby a woman has children with different men, has also increased. High father involvement and supportive coparenting may serve as barriers to multipartnered fertility. Using a subsample of the Fragile Families and Child Wellbeing Study (N=2363), we examined father involvement (measured as engagement, responsibility, and accessibility) and supportive coparenting as predictors of unmarried mothers' fertility. Discrete time survival analysis models indicated that mothers who perceived greater paternal engagement, responsibility, and supportive coparenting were more likely to have another child with the focal child's biological father, and less likely to have a child with a new man. Among noncoresidential mothers (mothers who were not living with the focal child's biological father), the same pattern of results emerged with one exception: paternal engagement did not predict either same or multipartnered fertility. Also, non-coresidential mothers that reported higher levels of accessibility, or contact between the focal child and the biological father, were more likely to have another child with him, and less likely to have a child with a new man. Overall, greater supportive coparenting and father involvement may decrease multipartnered fertility, even among non-coresidential parents.