Background: Behavioral support may be effective in increasing physical activity (PA) in school settings. However, there are no data collection systems to concurrently record PA and behavioral support. This paper describes the development and validation of the System for Observing Behavioral Ecology for Youth in Schools (SOBEYS)--an instrument used with existing observation systems to record PA within a behavioral ecological context. Methods: In 2013, experts created a set of behavioral categories to record prompting and reinforcement of PA during a recreational school activity program. The school provided supervision and equipment for lunchtime PA, making it possible to assess both PA and behavioral support. The system was implemented in spring semester 2014 at 1 suburban junior high school (N = 1452; 48% girls, 74% Caucasian) in the western Unites States. Results: Following multiple field trials, the SOBEYS instrument recorded behavioral categories of visual and verbal prompting, verbal, and nonreinforcement by adults and peers, and token reinforcement. Construct validity and acceptable interobserver agreement (>90%; kappa between 0.22 and 0.94) resulted. Conclusion: Trained SOBEYS users can accurately and reliably record the presence of behavioral support aimed at prompting and reinforcing PA in conjunction with established systematic observation instruments to record PA context and quantity.