Introduction: The recently updated UK physical activity (PA) and sedentary time (ST) guidelines recommend an average of at least 60 minutes of moderate to vigorous PA (MVPA) per day across the week and to minimise ST to reduce cardiovascular disease risk in 5-18 year olds. However, these guidelines are based primarily on evidence surrounding measurement of traditional cardiovascular disease risk factors (e.g., adiposity and blood biomarkers) rather than direct measures of arterial structure and function. Literature examining associations between PA or ST and vascular health in paediatric populations is of low quality due to few objective measures of PA and ST, small sample sizes and a lack of control of confounding variables. Aim: To examine associations between objectively measured PA and ST with vascular outcomes (endothelial function, arterial elasticity and arterial stiffness) and clustered cardiometabolic risk in children. Methods: Cross-sectional analysis of data from 4,277 children (2,226 girls) aged 10.6 ± 0.2 y from the Avon Longitudinal Study of Parents and Children. At age 9, a clustered cardiometabolic risk score was determined, vascular outcomes (flow mediated dilation for endothelial function, distensibility coefficient for arterial elasticity, pulse wave velocity for arterial stiffness) were measured age 10, and light PA, MVPA and ST measured via ActiGraph accelerometers age 11. Multiple linear regression analysis was combined with compositional data analysis for the accelerometer data to examine the associations between light or MVPA or ST as the predictor variable with the vascular variables or clustered cardiometabolic risk as the outcome. Missing data in covariates was replaced using multiple imputation. Models were adjusted for the following covariates: age, sex (group models only), age in years from peak height velocity, mother’s social class, baseline vessel diameter (flow mediated dilation models only), time between vascular outcome measurement and accelerometer measurement (or time between clustered cardiometabolic risk and accelerometer measurements when cardiometabolic risk was the outcome), cardiorespiratory fitness scaled to lean body mass, lean mass index, clustered cardiometabolic risk (except where the cardiometabolic risk score was the outcome) and family history of cardiovascular disease. Results: Neither light PA, or MVPA or ST (relative to the remaining activity behaviours) were significantly associated with any of the vascular outcomes. The proportion of time spent in MVPA and ST (relative to the remaining activity behaviours) was inversely (β=-0.126; 95% CI=-0.202 to -0.050; P=0.001) and positively (β=0.136; 95% CI=0.026 to 0.246; P=0.016) associated with clustered cardiometabolic risk in the group analysis and the relationships were sex dependent. Although MVPA was negatively associated with clustered cardiometabolic risk in both boys (β=-0.144; 95% CI=-0.255 to -0.034; P=0.011) and girls (β=-0.110; 95% CI=-0.211 to 0.009; P=0.032), only girls had a positive association between ST and clustered cardiometabolic risk (β=0.199; 95% CI=0.060 to 0.339; P=0.005). Conclusion: In childhood, MVPA should be maximised, and ST limited for cardiometabolic benefits. Longer exposure to cardiometabolic risk factors may be required to establish relationships between activity behaviours and vascular outcomes in youth. This highlights the need for additional high quality prospective research following individuals throughout adolescence and into early adulthood.