Autism spectrum disorder is a neurodevelopmental disorder estimated to have elevated prevalence in forensic populations (approximately 4.5%). It has been suggested that offenders with autism spectrum disorder engage more frequently in crimes against the person and sexual offences than other types of offences such as property, driving and drug offences. To date little is empirically known about the reasons why autistic individuals engage in sexual offences, yet understanding the motivation(s) for offending are key to developing and implementing effective interventions to help reduce both initial offending and also re-offending. In this study, semi-structured interviews were conducted with nine autistic sexual offenders in prisons and probation services across England and Wales. Thematic analyses revealed five main themes (social difficulties, misunderstanding, sex and relationship deficits, inadequate control and disequilibrium). Analyses indicated that social skills difficulties, lack of perspective/weak central coherence, misunderstanding the seriousness of their behaviours and a lack of appropriate relationships were the main reasons for offending reported by this group of autistic sexual offenders. Findings highlight a need to develop sex and relationship education interventions which are tailored to the needs of autistic individuals, to address both their reported reasons for offending and their reported lack of sexual knowledge and awareness.