Background Adverse childhood experiences (ACEs) are associated with multiple mental and physical health problems. Yet, mechanisms by which ACEs confer risk for specific problems are largely unknown. Children in foster care typically have multiple ACEs and high rates of negative sequelae, including delinquent behaviors. Mechanisms explaining this link have not been explored in this population. Impaired sleep has been identified as a potential mechanism by which ACEs lead to delinquency in adolescents, because inadequate sleep may lead to poor executive function and cognitive control – known risk factors for delinquency. Methods Interviews were conducted with 516 maltreated children in foster care, ages 9–11 years, and their caregivers regarding child exposure to ACEs, sleep problems, engagement in delinquent acts, symptoms of posttraumatic stress disorder, and current psychotropic medication use. ACEs data were also obtained from child welfare case records. Results After controlling for age, gender, race/ethnicity, placement type (residential, kin, foster), length of time in placement, posttraumatic stress symptoms, and current psychotropic medication use, sleep partially mediated the association between ACEs and delinquency. Conclusions Although delinquency is likely multiply determined in this population, improving sleep may be one important strategy to reduce delinquency.