Autistic adults report high stress and impaired ability to cope with stressors. Mindfulness-based stress reduction targets individual's own resources to regulate stress responses. An open feasibility study was conducted in an outpatient context, recruiting autistic adults without intellectual disability. The intervention, guided by trained mindfulness-based stress reduction teachers, followed the contents of the standard manual while adapting the delivery to autistic adults' needs. The total sample comprised of N = 50 participants; N = 43 (86%) attended at least one session, and N = 34 of these (79%) completed the intervention. Participants found mindfulness-based stress reduction logical, likely to lead to improvement and recommendable to autistic peers. No serious adverse events were reported. Measures of preliminary effectiveness indicated reduced symptoms of stress (p < 0.001; d = 0.51) and improved stress-coping skills (p = 0.017; d = 0.43) from pre- to post-intervention, as well as a reduction in symptoms of anxiety and depression. No changes were observed in mindful attention awareness, life satisfaction, or acceptance of the autism diagnosis. The data suggest that mindfulness-based stress reduction is a feasible and acceptable intervention for autistic adults in an outpatient setting. Further investigation in randomized clinical trials is warranted (Clinical Trials, clinicaltrials.gov, registration no. NCT05244265).