Issue AddressedApproximately 77% of NSW children aged 5 to 15 years do not meet physical activity guidelines and many spend a considerable amount of time sitting. Active breaks at primary school are feasible, may increase daily moderate to vigorous physical activity (MVPA) and decrease off‐task behaviour without adversely affecting cognitive function and learning.MethodsIn this quasi‐experimental study, 101 primary school children in six intervention classrooms participated in three 10‐minute active breaks per day for six‐weeks during class time, while five control classrooms were run as usual (n = 89). Physical activity levels were measured using wrist‐worn Actigraph wGT3X‐BT accelerometers and analysed using a random forest model. Students' off‐task behaviour, wellbeing, cognitive function and maths performance were also measured. School staff completed a brief feedback survey.ResultsChildren in the intervention group engaged in 15.4 and 10.9 minutes more MVPA per day at 3 and 6 weeks respectively (P < .001). Participation significantly increased the proportion of children who met the Australian 24‐Hour Movement Guidelines (P < .001). At pre, middle and end of intervention, 44.4%, 60.8% and 55.1% of intervention children and 46.5%, 45.9% and 45.8% of controls met the guidelines. Significantly fewer students engaged in off‐task behaviour in the intervention classes at mid and final weeks of intervention (−1.4 students, P = .003). No significant intervention effects were found for wellbeing, cognitive and maths performance.ConclusionsActive classroom breaks are an effective way to increase physical activity among primary school children while reducing off‐task classroom behaviour.So What?Primary school students' health would benefit from active breaks with no detrimental effects on wellbeing, maths and cognitive performance.