Objective The present study examined the daily, bidirectional relationships between sleep and mental health symptoms in youth presenting to mental health treatment. Methods Youth aged 6 to 11 (36% female, 44% European American) presenting to outpatient behavioral health treatment (N = 25) were recruited to participate in the study. Children and parents completed daily questionnaires regarding the child’s sleep, mood, and behavior for a 14-day period, while youth wore an actigraph watch to objectively measure sleep. Results Examining between- and within-person variance using multilevel models, results indicate that youth had poor sleep duration and quality and that sleep and mental health symptoms were highly related at the daily level. Between-person effects were found to be most important and significant bidirectional relationships exist. Conclusions Identifying and addressing sleep problems in the context of mental health treatment is important, as poor sleep is associated with increased symptomology and may contribute to worsened mental health.