We explore adolescents' and their parents' experiences of internet-based emotion regulation therapy for non-suicidal self-injury (NSSI). A qualitative study nested within a controlled feasibility trial. Online, semi-structured focus group interviews were conducted with outpatient adolescents with NSSI aged 13-17 years (n = 9) and their parents (n = 8) who had received therapist-guided Internet-delivered Emotion Regulation Individual Therapy for Adolescents (IERITA). Transcripts were analysed using reflexive thematic analysis. Three main themes were generated: (1) Fatigue - barriers to and during treatment, comprised of two sub-themes 'Arriving to services exhausted, needing motivation, and leaving feeling abandoned' and 'the burden of IERITA and the consequences of fatigue', (2) inter- and intrapersonal insights as facilitators of change and (3) Online, written contact with the therapist is beneficial and contributes with less pressure, comprised of three sub-themes 'the therapist behind the screen is essential', 'less pressure sitting alone: the physical absence of a therapist' and 'engaging on your own terms, in your own tempo'. Themes were consistent among adolescents and parents. Fatigue due to therapeutic engagement and previous help-seeking processes created barriers for engagement. Emotion regulation therapy was experienced as beneficial leading to inter- and intra-personal insights, facilitating change of maladaptive patterns. Therapists were regarded as indispensable, and the internet-based format did not hinder therapeutic alliance. The written format allowed for reflection and alleviated the pressure of relating to the therapist. Further research should explore experiences of other online treatment formats (e.g. synchronous or video-based) with regard to benefits, fatigue and therapist interaction.