Sleep-related problems (SRPs) are a common precursor to anxiety disorders, especially during peri-adolescence, and may be a predictor of treatment response. However, evidence-based anxiety treatments do not alleviate SRPs to a clinically significant degree. The current study examines whether improving sleep in a sample of young adolescents previously treated for anxiety disorders can further reduce anxiety severity. Participants include 46 adolescents (65% female, ages 9–14 years) previously treated for anxiety disorders who self-selected to participate in a 6-week open trial of targeted sleep enhancement (TIGERS). Measures of parent- and child report on SRPs, and clinician-rated anxiety severity, were collected at several time points of the study: pre-, and postanxiety treatment, pre- and post-TIGERS, and at four yearly follow-ups. Two hierarchical mediation models were run using either parent- or child report on sleep. On the between-subjects level, participants with higher overall average SRPs also had higher overall anxiety severity over all time points. On the within-subjects level, participation in TIGERS was not directly associated with change in anxiety severity—however, participation in TIGERS was associated with a drop in SRPs, which was associated with a drop in anxiety severity. Improving sleep in anxious peri-adolescents further improves anxiety above and beyond anxiety treatment. Further research is needed to confirm the effects of improving sleep on clinical anxiety in a randomized controlled trial.