Post-diagnostic support for autistic adults in the United Kingdom is geographically inequitable and, in general, considered inadequate. This results in autistic adults facing unnecessary challenges and wide-ranging poor outcomes. A modified Delphi study sought to establish autistic adults' priorities for optimal provision of post-diagnostic support. The study was co-designed with 10 autistic adults across a series of three online workshops. Forty-three autistic adults, who had received their diagnosis in adulthood, completed a series of three online questionnaires through which 153 support priorities were identified. In a fourth questionnaire round, completed by 139 autistic adults, 24 priorities reached or exceeded a consensus threshold of 80% agreement that the item specified an important element of post-diagnostic support. Participants ranked "access to support where they live, training of professionals, support to process the impact of a late diagnosis, use of their preferred mode of contact and an individualised support plan" as their key priorities for post-diagnostic support. The whole priority list provides a clear and practical articulation of what autistic adults want from post-diagnostic support. This should be used by policymakers and service providers in the United Kingdom to ensure that post-diagnostic support for autistic adults is fit for purpose.