Parent-mediated intervention is widely used for pre-schoolers with autism spectrum disorder (ASD). Previous studies indicate small-to-moderate effects on social communication skills, but with a wide heterogeneity that requires further research. In this randomized controlled trial (RCT), cooperative parent-mediated therapy (CPMT) an individual parent coaching program for young children with ASD was administered to preschool children with ASD. All children received the same low-intensity psychosocial intervention (LPI) delivered in community settings, to evaluate the potential additional benefit of CPMT. Thirty-four participants with ASD (7 females; 27 males; aged 2, 6, 11 years) and their parents were included in the trial. The primary blinded outcome was social communication skills, assessed using the ADOS-G social communication algorithm score (ADOS-G SC). Secondary outcomes included ASD symptom severity, parent-rated language abilities and emotional/behavioral problems, and self-reported caregiver stress. Evaluations were made at baseline and post-treatment (at 6 months) by an independent multidisciplinary team. Results documented that CPMT showed an additional benefit on LPI with significant improvements of the primary blinded outcome, socio-communication skills, and of some secondary outcomes such as ASD symptom severity, emotional problems and parental stress related to parent-child dysfunctional interaction. No additional benefit was found for language abilities. Findings of our RCT show that CPMT provide an additional significant short-term treatment benefit on ASD core symptoms, when compared with active control group receiving only LPI.