Increased reactivity to daily stressors is associated with mental health difficulties, which are common in autistic individuals. We investigated affective reactivity to daily-life stress, cognitive emotion regulation, and their link with co-occurring mental health symptoms in adolescents and young adults with autism. A 6-day ecological momentary assessment protocol was used to assess perceived daily-life stress (event-related, activity-related, and social stress) as well as negative affects in autistic (n=39, age=18.4) and non-autistic (n=55, age=18.1) participants. Co-occurring mental health difficulties, social functioning, and cognitive emotion regulation were assessed with questionnaires and clinical evaluations. Youth with autism showed higher levels of perceived stress related to their daily social context and activities, as well as an increased affective reactivity to activity-related stressors compared with non-autistic youth. Moreover, they reported using less adaptive and more non-adaptive emotion regulation, the latter increasing their affective response to daily stressors and possibly contributing to the severity of co-occurring mental health symptoms. Our findings demonstrate high perceived daily-life stress in autistic adolescents and young adults. To minimize the negative effects of stress and the development of mental health symptoms, interventions could focus on effective stress management and strategies that autistic young people use to manage their emotions.