Abstract: We apply family systems theory and the kinscripts framework to advance understanding of cohabiting stepfather involvement and kin work in low-income Black families raising children, from men’s own perspectives. Analysis of in-depth interviews with 15 cohabiting stepfathers revealed three central kin work domains: involvement in child discipline, taking on financial responsibilities for the child, and developing and maintaining a paternal relationship to the child in the context of a complex family. In each domain, participants described processes of negotiating their involvement with their partners. They also described often negotiating with their stepchildren and sometimes also with the child’s biological father. In multifather family systems, cohabiting stepfathers reported acknowledging, accepting, and adapting their involvement to the reality that there was more than one father to the child. Our analysis provides a theoretical advance in understanding men’s kin work, social fatherhood, and complex families.