This study examined the effectiveness of ‘Emotion-Based Social Skills Training (EBSST)’ a manualised social-emotional intervention designed to improve emotional competence in school-aged children with Autism Spectrum Disorder (ASD). Participants were 217 children (aged 7-13 years) with ASD without Intellectual Disability attending 41 mainstream primary schools in NSW Australia. Data on emotional competence, social skills and mental health difficulties were collected using teacher and parent informant report questionnaires in a pre-test/post-test control group design. One hundred and six students took part in the treatment and 111 students were in the control group. School Counsellors delivered the 16 session treatment to groups of 3-8 students in their schools. Teachers and parents also received six sessions of EBSST in separate groups. Participants received a booster session at six months follow-up. EBSST improved teacher reported emotional competence as measured by the Emotions Development Questionnaire (EDQ). The effect size was large and improvements were sustained at 6 months follow-up. Parent reported emotional competence and more general measures of social skills and mental health were insensitive to change across informants. This study has important implications for students, teachers and parents and provides a valuable basis for further research and development of EBSST and the EDQ.