Guided by a dyadic view of children's peer behavior, this study assessed actor and partner effects of attachment security and temperament on young children's behavior with an unfamiliar peer. At 33 months of age, child-mother attachment security was assessed via a modified Strange Situation procedure, and parents reported on child temperament (anger proneness and social fearfulness). At 39 months, same-sex children (N = 114, 58 girls) were randomly paired, and child dyads were observed during 3 laboratory visits occurring over 1 month. Actor-partner interdependence models, tested via multilevel modeling, revealed that actor security, partner anger proneness, and acquaintanceship (e.g., initial vs. later visits) combined to predict child behavior. Actor security predicted more responsiveness to the new peer partner at the initial visit, regardless of partner anger proneness. Actor security continued to predict responsiveness at the 2nd and 3rd visits when partner anger was low, but these associations were nonsignificant when partner anger was high. Actor security also predicted a less controlling assertiveness style at the initial visit when partner anger proneness was high, yet this association was nonsignificant by the final visit. The findings shed light on the dynamic nature of young children's peer behavior and indicate that attachment security is related to behavior in expected ways during initial interactions with a new peer, but may change as children become acquainted.