Objective: Many school-aged children do not meet the daily minimum recommendations for accruing moderate-to-vigorous physical activity (MVPA), and spend much of their day sedentary. The purpose of this study was to investigate the effects of teacher-delivered classroom-based physical activity (CBPA) on students' MVPA and sedentary behaviors. Methods: Participants included 157 students across 7 classrooms and 500 student-days of observation. Students wore accelerometers for one week during fall of 2017, and teachers recorded their CBPA offerings daily. Minutes of scheduled recess and physical education (PE) also were recorded. Results: Overall, students spent the majority of the school day engaged in sedentary behavior, and accrued an average of 20 and 28 minutes of MVPA on non-PE and PE days, respectively. Students did not engage in lengthy bouts of sedentary behavior, and spent approximately 30 minutes each day in sedentary bouts. Mixed-effects regression analyses revealed that offering any CBPA was associated with greater percent time in MVPA and less in sedentary behavior. Conclusion: CBPA is an important contributor to the 30 minutes of school-day MVPA that students should accrue, especially since PE and recess are often not sufficient.