Schools are in a unique position to ensure that all students meet the current physical activity (PA) recommendations. This study aimed to examine 1st to 3rd grade elementary students’ accelerometer measured school day PA in the United States (U.S.) and Finland. Methods: The sample consisted of 200 students (107 girls, 93 boys; ages 6 to 8) and their school day PA was monitored with hip-worn ActiGraph GT3X+ accelerometers across a 5-day school week and the thresholds 100 and 2296 count per minute were used to separate sedentary time, light PA, and moderate-to-vigorous PA (MVPA). Results: On an average school day, students were engaged in MVPA for 20.0 min in the U.S. and 24.1 min in Finland. Students’ school-day MVPA was 9 to 16 minutes higher during physical education (PE) days compared with non-PE days (U.S: 25.8 vs. 16.6 min/day; Finland: 36.3 vs. 20.1 min/day). Girls had less MVPA and more sedentary time compared with boys in both samples. Conclusion: This study highlights both the role of PE and other school day physical activities in meeting PA guidelines. Policy measures are needed to change the structure of the school day and enhance PA to ensure that students meet the PA recommendations.