Background: Education outside the classroom (EOtC) is a curriculum-based approach to teaching that has shown positive associations with children's physical activity and academic learning in small-scale case studies. The purpose of this large-scale quasi-experimental study was to determine if children who participate regularly in EOtC spend more time being physically active than children who do not. Methods: In the 2014/2015 study TEACHOUT, classes were recruited in pairs such that each EOtC class had a non-EOtC comparison class at the same school and grade level. Participants in 17 EOtC classes and 16 comparison parallel classes across Denmark wore an Axivity AX3 accelerometer taped to the lower back for seven consecutive days. Data from 201 EOtC participants (63.3% girls, age 10.82 ± 1.05,) and 160 comparison participants (59.3% girls, age 10.95 ± 1.01) were analysed using an 'intention to treat' (ITT) approach. The amount of EOtC the participants were exposed to was monitored. Associations between time spent in different physical activity intensities and EOtC group and sex were assessed using generalised linear models adjusted for age. In a second analysis, we modified the sample using a 'per protocol' (PP) approach, only including EOtC and comparison class pairs where the EOtC class had >150 min and the comparison had <150 min of EOtC during the measured week. Results: On average, EOtC participants spent 8.4 (ITT) and 9.2 (PP) minutes more in moderate-to-vigorous physical activity (MVPA) per day than comparison participants (p < 0.05). However, EOtC boys spent 18.7 (ITT) and 20.8 (PP) minutes more in MVPA per day than comparison boys (p < 0.01), while there were no significant between-group differences for girls. Conclusions: For boys, EOtC was associated with more daily time being spent moderately and vigorously physically active. No differences were observed for girls. Implementing EOtC into schools' weekly practice can be a time- and cost-neutral, supplementary way to increase time spent in PA for boys through grades three to six. Trial registration: The Scientific Ethical Committee in the Capital Region of Denmark protocol number H-4-2014-FSP. 5 March, 2014.