Although epilepsy commonly presents with autism in children, it is currently unknown whether established estimates represent the prevalence and incidence of epilepsy in autistic adults. Our objective was to use population-level Medicaid data to determine prevalence, incidence, and antiepileptic drug use associated with epilepsy in a unique population of autistic adults aged 21+ with (N = 2738) and without (N = 4775) intellectual disability and to compare results to adults with intellectual disability alone (N = 18,429). We found that 34.6% of autistic adults with intellectual disability and 11.1% of autistic adults without intellectual disability had epilepsy, compared to 27.0% of adults with intellectual disability alone. New incidence of epilepsy was 23.6 incident cases per 1000 person years (95% confidence interval: 21.3, 26.2) in autistic adults with intellectual disability, 7.7 incident cases per 1000 person years (95% confidence interval: 6.6, 8.9) in autistic adults without intellectual disability, and 15.9 incident cases per 1000 person years (95% confidence interval: 15.2, 16.7) in adults with intellectual disability alone. Female sex and intellectual disability were associated with increased risk of prevalent and incident epilepsy in autistic adults. Findings underscore the importance of treating prevalent epilepsy and screening and preventing incident epilepsy in autistic adults as they age.