Sufficient summer/autumn vitamin D status appears important to mitigate winter nadirs at northern latitudes. We conducted a cross-sectional study to evaluate autumn vitamin D status and its determinants in 782 Danish 8–11-year-old children (55°N) using baseline data from the Optimal well-being, development and health for Danish children through a healthy New Nordic Diet (OPUS) School Meal Study, a large randomised controlled trial. Blood samples and demographic and behavioural data, including 7-d dietary recordings, objectively measured physical activity, and time spent outdoors during school hours, were collected during September–November. Mean serum 25-hydroxyvitamin D (25(OH)D) was 60·8 (SD 18·7) nmol/l. Serum 25(OH)D levels ≤50 nmol/l were found in 28·4% of the children and 2·4% had concentrations <25 nmol/l. Upon multivariate adjustment, increasing age (per year) (β −2·9; 95% CI −5·1, −0·7 nmol/l), female sex (β −3·3; 95% CI −5·9, −0·7 nmol/l), sampling in October (β −5·2; 95% CI −10·1, −0·4 nmol/l) and November (β −13·3; 95% CI −17·7, −9·1), and non-white ethnicity (β −5·7; 95% CI −11·1, −0·3 nmol/l) were negatively associated with 25(OH)D (all P<0·05). Likewise, immigrant/descendant background was negatively associated with 25(OH)D, particularly in females (β −16·3; 95% CI −21·9, −10·7) (P<0·001) (Pinteraction=0·003). Moderate-to-vigorous physical activity (MVPA) (min/d) (β 0·06; 95% CI 0·01, 0·12), outdoor walking during school hours (min/week) (β 0·4; 95% CI 0·1, 0·6) and intake of vitamin D-containing supplements ≥3 d/week (β 8·7; 95% CI 6·4, 11·0) were positively associated with 25(OH)D (all P<0·05). The high proportion of children with vitamin D status below the recommended sufficiency level of 50 nmol/l raises concern as levels expectedly drop further during winter months. Frequent intake of vitamin D supplements was strongly associated with status. MVPA and outdoor activity during school hours should be investigated further in interventions to improve autumn vitamin D status in children at northern latitudes.