This study aimed to assess whether the transdiagnostic therapy elements of an online cognitive behaviour therapy anxiety program also impact on sleep-related problems (SRPs) in anxious youth. Participants were drawn from two previously published studies evaluating online cognitive behaviour therapy for child anxiety (BRAVE-ONLINE). The study included 63 children 7–12 years of age (M = 9.49, SD = 1.37) and 71 adolescents 12–18 years of age (M = 13.90, SD = 1.68). SRPs, severity of anxiety diagnosis, anxiety symptoms, number of diagnoses, depressive symptoms, and global functioning were assessed at pre-, post-, and 6-month follow-up assessment points. SRPs were positively related to anxiety symptoms and severity for children and were positively related to depression for adolescents. SRPs did not differ between male and female participants, between children and adolescents, or between those who had generalised anxiety disorder in their profile and those who did not. Finally, children but not adolescents participating in the online program demonstrated a significantly greater reduction in SRPs from pre- to posttreatment compared to the waitlist group, and these gains were maintained at 6-month follow-up. Treatment focusing on child anxiety alone may reduce SRPs in children but not adolescents. Although further research is clearly needed, clinicians should ensure that they assess for SRPs in their teenage clients and directly target SRPs in treatment where required.