Objective/background. Adolescents with attention-deficit/hyperactivity disorder (ADHD) experience greater difficulties in the domains of sleep, daytime sleepiness, and functioning compared to their peers. However, the relationship between these domains has not been fully elucidated. This study aimed to examine the relationship between sleep problems (including daytime sleepiness), ADHD severity, and functional outcomes (irritability, sluggish cognitive tempo, homework difficulties, and substance use) in a sample of adolescents with ADHD. Patients/methods. Eighty-two adolescents (13–17 years) and their families participated in the study. Sleep was measured by both adolescent and parent-report. Adolescent irritability and sluggish cognitive tempo were reported by both adolescents and parents, while other variables were reported by a single reporter (homework difficulties – parent; ADHD severity – parent; substance use – adolescent). Analyses controlled for demographic factors and internalising and externalising comorbidities. Results. A weak relationship was found between adolescent-reported sleep problems and daytime sleepiness, which became non-significant in adjusted analyses (β = −0.19, p = 0.115). In adjusted analyses, there was an association between adolescent-reported sleep problems and adolescent-reported irritability (beta = −0.27, p = 0.023) as well as between adolescent-reported daytime sleepiness and parent-reported sluggish cognitive tempo (beta = 0.28, p = 0.033). In adjusted analyses, parent-reported adolescent sleep problems were associated with ADHD severity (beta = 0.54, p = <0.001), parent-reported sluggish cognitive tempo (beta = 0.64, p = <0.001), both reporters of irritability (parent-report beta = 0.32, p = 0.004; adolescent-report beta = 0.29, p = 0.022), and homework problems (beta = 0.37, p = 0.003). Parent-reported daytime sleepiness was associated with parent-reported sluggish cognitive tempo (beta = 0.34, p = 0.024). Conclusions. This study demonstrates the importance of a holistic assessment of adolescents with ADHD, not only focusing on symptomatology but also on sleep problems and functional outcomes. The importance of multi-informant assessment of sleep problems is also reinforced.