Background: The aim of this feasibility study was to adapt and model a behavioural intervention for anxiety with autistic adults with moderate to severe intellectual disabilities. Method: Twenty-eight autistic adults with moderate or severe intellectual disabilities, 37 carers, and 40 therapists took part in this single-group non-randomised feasibility study designed to test intervention feasibility and acceptability, outcome measures, and research processes. Results: The intervention was judged as feasible and acceptable by autistic adults with intellectual disabilities, carers, and therapists. Minor intervention revisions were suggested. Carers completed 100% of outcome measures and the missing data rate was low. Complying with legislation governing the inclusion of participants who lack capacity to decide whether they wanted to take part in this study led to an average 5-week enrolment delay. Conclusion: The intervention and associated study processes were judged to be feasible and acceptable and should now be tested within a larger randomised trial.