The independent associations of physical activity (PA), sleep duration (SL), and screen time (ST) with health outcome measures have been well-documented among children and adolescents, but not in youth with moderate/severe ADHD. To this end, the present study aimed to investigate the associations between three components within 24-hour movement behaviour (24-HMB) framework and core symptoms and school engagement in youth with moderate/severe ADHD. This study used pooled data from the 2021-2022 U.S. National Survey of Children's Health, which included a nationally representative sample of 4797 youth aged 6-17 years. Logistic/Ordinal regressions examined associations between meeting 24-HMB guideline combinations and core symptoms and school engagement among youth with moderate/severe ADHD, adjusting for age, sex, family income, body mass index, and ethnicity. Only 6.9 % of youth with moderate/severe ADHD met all three 24-HMB guidelines. Compared to those meeting no guidelines, youth meeting all guidelines was beneficially associated with inattention symptoms (self-regulation: OR = 0.44, 95%CI:0.35-0.57; concentration: OR = 1.26, 95%CI:1.01-1.55) and better performance of school engagement (caring about doing well: OR = 0.52, 95%CI:0.41-0.65; completing homework: OR = 0.49, 95%CI:0.39-0.62). Meeting the ST + SL guidelines was associated with significantly lower odds of hyperactivity/impulsivity symptoms (argument, OR = 1.33, 95 % CI: 1.12-1.58; staying calm, OR = 0.59, 95 % CI: 0.46-0.75) compared to those who met none of guidelines. Meeting 24-HMB guidelines was associated with lower likelihood of ADHD core symptoms and greater likelihood of school engagement. Integrative "whole day" behavioural interventions warrant consideration for improving school engagement-related outcomes and reducing ADHD core symptoms in this vulnerable population.