This study aimed to evaluate whether the Therapist-assisted Online Parenting Strategies programme increased parenting behaviours known to be supportive of adolescents experiencing anxiety and/or depression. Secondary parenting outcomes of parental self-efficacy, parental accommodation, carer burden, parent-adolescent attachment, family functioning and parent distress were also examined, along with adolescent outcomes of anxiety and depression symptoms, suicidal ideation and sleep. Seventy-one parents (94.4% females) and their adolescents (73.2% females) aged 12-18 years (Mean = 15.02, SD = 1.56) being treated for depression and/or anxiety in Australia were recruited into a single-arm double-baseline open-label trial. Parents received Therapist-assisted Online Parenting Strategies, which comprised up to nine web-based modules each supplemented with coaching sessions via videoconferencing. Outcomes were analysed using latent growth curve modelling to determine if changes to outcomes at post-intervention (4 month post-second baseline) exceeded changes between two baselines measured 1 month apart. Sixty-five parents (91.6%) completed at least one module of the online parenting intervention and on average received nine coaching sessions (SD = 2). Parenting behaviours targeted by Therapist-assisted Online Parenting Strategies improved at post-intervention (Cohen's d = 1.16, 95% confidence interval [0.78, 1.51]). Parent-reported parental self-efficacy and parent-adolescent attachment increased (Cohen's d = 1.44 [1.05, 1.82] and 0.39 [0.05, 0.74], respectively), while impairments to family functioning and parent distress decreased (Cohen's d = -0.51 [-0.86, -0.16] and -0.84 [-1.23, -0.44], respectively). Changes to adolescent anxiety, depression and sleep were not significant. The Therapist-assisted Online Parenting Strategies intervention improved self-reported parenting behaviours, parental self-efficacy, parent levels of distress, parent-adolescent attachment, and family functioning in parents with adolescents being treated for anxiety and/or depression. However, significant changes in adolescent mental health and sleep outcomes at post-intervention were not observed. The usefulness of a therapist-supported online parenting programme in addressing a service gap for parents seeking professional help is indicated. Australian New Zealand Clinical Trials Registry Number (ACTRN) 12618000290291, prospectively registered on 26 February 2018; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=368031.