Objectives <p>Elevated stress levels in autistic adolescents may significantly hinder their quality of life and development. Stress Autism Mate (SAM) Junior, a mobile self-help tool, was designed in co-creation with adolescents with autism to enhance stress recognition and reduce daily stress. To improve its effectiveness, this qualitative study examined user experiences with the app, focusing on identifying factors that may influence users’ willingness to begin and continue its use.</p> Methods <p>A total of 15 adolescents with autism (12 girls, 3 boys) aged between 12 and 18&#xa0;years were recruited from three Dutch mental health care facilities. They were asked to use the SAM Junior app daily for at least 1 week before taking part in a 30-min semi-structured interview. Utilizing the User Experience Technology Acceptance Model as a theoretical framework, deductive thematic analysis was performed on four main themes: perceived usefulness, perceived ease of use, hedonic quality, and health improvement.</p> Results <p>While user experiences varied substantially, participants generally expressed moderate satisfaction with the app’s stress-reducing potential. Several features could be viewed as ambiguous, frustrating, and stress inducing, while some could also be overlooked. These hindrances limited both the app’s use and perceived effectiveness.</p> Conclusions <p>Overall, the SAM Junior app was perceived as promising but flawed in its current form. The participants’ feedback demonstrated how autistic traits can contribute to technostress and shape user experiences, offering multiple directions to improve the app and similar mHealth interventions for autistic adolescents.</p>