Parental reflective functioning (PRF) plays a protective role in the development of children with histories of early adversity, including adopted children. This is the first study to investigate the developmental trajectories of PRF and children’s socio-emotional problems in the first 4 years after international adoption (N = 48 families, mean age (T1) = 20.7 months) and to examine the mediating role of parenting stress in the relation between PRF and child socio-emotional problems. Multilevel modeling indicated that age at adoption and parent gender moderated the development of PRF and child socio-emotional problems. Moreover, decreases in PRF were associated with more socio-emotional problems in the children. These relations were mediated by parenting stress, and particularly feelings of incompetence and marital dissatisfaction.