BackgroundNonsuicidal self-injury (NSSI) is common in adolescence and is associated with several adverse outcomes. Despite this, few established treatment options exist. Online treatment seems promising for several conditions; however, knowledge on NSSI is scarce. It is important to explore how online treatment for NSSI is experienced to improve such interventions and learn more about factors that are important in the treatment of adolescents with NSSI.ObjectiveThis study aims to explore the experiences of a novel online treatment for adolescents with NSSI and their caregivers.MethodsA qualitative study using thematic analysis was conducted through semistructured interviews with 9 adolescents and 11 caregivers at treatment termination or at the 6-month follow-up of the online emotion regulation individual therapy for adolescents.ResultsA total of 3 overarching themes were identified. The theme support can come in different shapes showed how support could be attained through both interaction with the therapist as well as through the format itself (such as through the fictional characters in the material and the mobile app). Caregivers found it helpful to have their own online course, and adolescents accepted their involvement. The theme self-responsibility can be empowering as well as distressing showed that self-responsibility was highly appreciated (such as deciding when and how to engage in treatment) but also challenging; it caused occasional distress for some. The theme acquiring new skills and treatment effects showed the advantages and challenges of learning several different emotion regulation skills and that decreased emotion regulation difficulties were important treatment outcomes for adolescents. In addition, several different skills seemed to facilitate emotion regulation, and having access to such skills could hinder NSSI.ConclusionsOnline emotion regulation individual therapy for adolescents seems to offer an accepted way to deliver family interventions for this target group; facilitate skills training with several means of support, including support from both the mobile app and the therapist; contribute to decreasing emotion regulation difficulties and teaching skills that could hinder NSSI; and cause (in some individuals) distress because of the self-responsibility that is inherent to online formats, which needs to be addressed.