Objectives: Children with ADHD commonly exhibit sleep disturbances, but there is limited knowledge about how sleep and sleep timing are associated with cognitive dysfunction in children with ADHD. Methods: Participants were 350 children aged 5 to 12 years diagnosed with ADHD. Three sleep-related constructs--time in bed, social jetlag (i.e., discrepancy in sleep timing pattern between school nights and weekend nights), and sleep disturbances were measured using a caregiver-report questionnaire. Linear regression models assessed the associations between sleep-related constructs and cognitive performance. Results: After adjustment for sociodemographic variables, there were few associations between time in bed or sleep disturbances and cognitive performance, however, greater social jetlag was negatively associated with processing speed ([beta] = -0.20, 95% CI [-0.35, -0.06]), visually-based reasoning ([beta] = -0.13, 95% CI [-0.27, 0.00]), and language-based reasoning ([beta] = -0.22, 95% CI [-0.36, -0.08]); all p < 0.05). Conclusion: Social jetlag, but not time in bed or disturbances, was associated with lower cognitive performance among children with ADHD.