Caregivers can experience significant challenges following their child's autism diagnosis and often seek informational, relational and emotional support. Post-diagnostic support for caregivers has received relatively little research attention and represents a significant gap in the international evidence base. We used an iterative codesign process to develop a manualised group-based post-diagnostic programme, Empower-Autism, with associated theory of change, to address the diverse needs of caregivers and improve caregiver mental health. Empower-Autism blended evidence-informed autism psychoeducation with psychotherapeutic components grounded in Acceptance and Commitment Therapy. Three feasibility groups (two in-person and one online) were delivered with 29 clinically referred, socioeconomically diverse caregivers. We evaluated feasibility and acceptability through quantitative attendance and satisfaction data, and thematic analysis of detailed interviews with 17 participants and 5 facilitators. Attendance was satisfactory (76% participants meeting prespecified criteria) and programme satisfaction was good. Qualitative analysis suggested the programme was acceptable and accessible, and highlighted the areas for improvement. Caregivers described perceived benefits from the programme, including improved wellbeing, social connection, autism positivity and more attuned parenting. EmpowerAutism is being evaluated within a large randomised controlled trial and, if shown to be clinically and cost-effective, this programme will fill an identified evidence gap in the United Kingdom and global provision.