Autism diagnostic assessment for children and young people tends to focus on within-individual ‘deficits’ and non-autistic interpretation of observable behaviours. In contrast, a neurodiversity-affirmative assessment involves centring autistic perspectives, focusing on strengths, differences and needs, and acknowledging the role of the environment. The present study was designed to focus on professionals’ perspectives on neurodiversity-affirmative autism diagnostic assessment. Autism community stakeholders were consulted on the design of the interview schedule. Semi-structured interviews were conducted with 19 professionals aged 30-73 years who work in a multidisciplinary team within the UK. Reflexive thematic analysis was used to identify themes. Professionals described a holistic, balanced assessment involving assessing individual and contextual strengths and needs. Our findings suggest that assessing strengths could be beneficial for autistic children and young people’s long-term wellbeing. Professionals also highlighted an autistic person-centred assessment and a strengths-and-needs-led, rather than diagnosis-led approach. These findings highlight the importance of a holistic, person-centred, strengths-and-needs-led assessment that has the potential to support autistic thriving.