Young people with autism have support needs related to self-determination that are currently not adequately addressed in Spain. This study aimed to assess the effectiveness, implementation, and acceptability of a program to support self-determination for young adults with autism in Spain. A multicenter, mixed methods randomized controlled trial (RCT) study was conducted (2020-2022). Young adults with autism between 17 and 30 years of age were recruited, and 40 were randomly assigned to the intervention or waiting-list group. Quantitative and qualitative data on outcomes relating to self-determination, program implementation and acceptability were collected at baseline, during, and after the intervention using several tools, including the SDI: SR and focus groups. Joint displays were used to integrate the quantitative and qualitative results for a comprehensive evaluation of the program. The quantitative results revealed no significant differences between the intervention and waiting list groups but indicated positive impacts on agentic actions and overall self-determination as reported by participants and caregivers. The qualitative results expanded the quantitative results by identifying personal and contextual barriers and facilitators of self-determination while offering deeper insights into the quantitative outcomes. Implementation fidelity was high, and the qualitative data provided areas for improvement and identified challenges and best practices. Program acceptability was high, and the group format proved useful. Our study provides the first empirical evidence of an intervention designed to promote self-determination in individuals with autism in Spain, but further research is needed. This trial was retrospectively registered at www.clinicaltrial.org (NCT05938751) on January 1