AbstractChildren with motor coordination problems are at increased risk of both sedentary behaviour and internalizing problems relative to their typically developing (TD) peers. The purpose of this study was to examine the moderating role of sedentary behaviour on the relationship between motor skill proficiency and anxiety/depression scores in children at risk for Developmental Coordination Disorder (DCDr) and a sample of typically developing (TD) children. Data for the present study were derived from the Coordination and Activity Tracking in CHildren (CATCH) cohort study. 507 children aged 4-5 years (219 girls, 288 boys, mean age: 59.3 months) were classified as TD (>16th percentile), or DCDr (?16th percentile) based on Movement Assessment Battery for Children 2nd edition scores. Sedentary and physical activity behaviour was measured using an Actigraph GT3X+ activity monitor device. Parent-reported Child Behaviour Checklist (CBCL) scores were used to assess internalizing problems (i.e. anxiety/depression). Multiple linear regression (moderation) analysis showed sedentary behaviour significantly moderated the relationship between motor skill proficiency and anxiety/depression scores after adjusting for sex and physical activity behaviour, R2 change = .01, F (1,501) = 4.13, p = .043. Johnson-Neyman technique revealed DCDr children begin to report significantly higher anxiety/depression scores when their sedentary behaviour is at least one minute per day above the group mean (M = 452 mins/day). Above average levels of sedentary behaviour exacerbate internalized problems among DCDr children. Interventions should focus on incorporating strategies to reduce sedentary behaviour in order to buffer anxiety/depressive symptoms for children with poor motor skill proficiency.