This RCT study examined efficacy of a preschool peer social intervention (PPSI) in facilitating social engagement of preschoolers with high-functioning ASD (HFASD; N = 65). HFASD participants were randomly assigned by preschool to a 6-month intervention (play, interaction, or conversation) or a waitlisted-treatment-as-usual control group. Trained on-site therapists led the PPSI in preschools, in small (n = 3-4) mixed (HFASD/typical) groups. Results showed that all intervention groups improved over time, each mainly in its own targeted peer-engagement domain, but the control group even deteriorated on some measures. Intervention groups also showed generalization to untrained domains (adaptive skills) and settings (play complexity during preschool activities). It is advised that individualized needs-based holistic peer intervention, comprising all three domains, should be part of early ASD intervention.