Background: Physical activity (PA) is decreasing among children, while sedentary behavior (SB) is increasing. Schoolyards seem suitable settings to influence children's PA behavior. This study investigated the associations between schoolyard characteristics and moderate-to-vigorous physical activity (MVPA) and SB of children aged 8-11 years at schoolyards. Methods: Twenty primary schools in the Netherlands were involved. A total of 257 children wore an accelerometer and global positioning system (GPS) device for 5 consecutive days to objectively assess their PA levels and presence at the schoolyard, respectively. Accelerometer and GPS data were merged using the personal activity and location measurement system. Multilevel linear regression analyses were used to study correlates of MVPA and SB at schoolyards. Results: On average, children spent 54 minutes a day at the schoolyard, 9 minutes of which were spent in MVPA and 20 minutes in SB. Boys engaged in MVPA longer than girls at the schoolyard. Fixed equipment, such as high bars and soccer goals, teacher-initiated activities, and the presence of a ball games policy were correlates of more MVPA and less SB. Conclusion: Well-designed schoolyards, including PA-enhancing fixed equipment, a supportive PA climate created by teachers, and supportive schoolyard policies may contribute to increased PA and decreased SB during school recess among school-aged children.