--- - |2 Abstract Interactions with mothers and fathers form a critical foundation for youths’ later peer relationships. This study focused on the associations of maternal and paternal sensitivity in third grade with friendship outcomes in sixth grade. Because interactions with parents provide the earliest opportunities for children to learn prosocial behaviors such as cooperation, and cooperation is theoretically conceptualized as a foundation of friendship, cooperation may play a role in the link between parenting and children’s later social relationships. Using the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development sample (N = 955; 51% female; 83% White), and a multimethod, multireporter longitudinal design, children’s cooperation was tested as a mediating mechanism that linked parental sensitivity to early adolescent friendship. Results indicated that most indirect paths from both mothers and fathers’ sensitivity to observed friendship interactions and perceived friendship quality, via cooperation, were significant. Findings provide a more concrete understanding of how parental sensitivity translates to better friendships. - Social Development, EarlyView.