ObjectivesThe aim of this study was to test the feasibility, acceptability and preliminary effects of a recently developed smartphone application, Sleep Ninja, for adolescent sleep difficulties.SettingThe study was conducted online with Australian individuals recruited through the community.ParticipantsParticipants were 50 young people aged 12–16 years with sleep difficulties.DesignA single-arm pre–post design was used to evaluate feasibility, acceptability and sleep and mental health variables at baseline and postintervention.InterventionCognitive–behavioural therapy for insomnia informed the development of the Sleep Ninja. The core strategies covered by the app are psychoeducation, stimulus control, sleep hygiene and sleep-related cognitive therapy. It includes six training sessions (lessons), a sleep tracking function, recommended bedtimes based on sleep guidelines, reminders to start a wind-down routine each night, a series of sleep tips and general information about sleep. Users progress through each training session and conclude the 6-week programme with a black belt in sleep.Outcome measuresFeasibility was evaluated based on consent rates, adherence and attrition, acceptability was assessed using questionnaires and a poststudy interview, and sleep, depression and anxiety variables were assessed at baseline and postintervention.ResultsData indicated that the Sleep Ninja is a feasible intervention and is acceptable to young people. Findings showed that there were significant improvements on sleep variables including insomnia (within-group effect size d=−0.90), sleep quality (d=−0.46), depression (d=−0.36) and anxiety (d=−0.41).ConclusionsThe Sleep Ninja is a promising intervention that could assist adolescents who experience sleep difficulties. A follow-up randomised controlled trial is now warranted.Trial registration numberACTRN12617000141347