Children's abilities to regulate their behaviors are critical for learning and development, yet researchers lack an objective, precise method for assessing children's behavioral regulation in their everyday environments such as their classrooms. This study tested a sensor-based approach to assess preschool children's behavioral regulation objectively, precisely, and naturalistically. Children wore accelerometer devices as they engaged in center-based play in their preschool classrooms for roughly 45 min (N = 50 children, 48% female, mean age = 4.5 years). Set to record data each second, these devices collected information about children's movement (N = 140,564 observations). From these data, the authors extracted concrete behaviors hypothesized to index behavioral regulation and compared them with teacher and observer ratings of the same. Initiating movement more frequently, staying seated in activities for shorter amounts of time, and spending a greater amount of time in motion were related to lower ratings of attention and inhibitory control by teachers and by observers of classroom group time, median r = 0.45, p < 0.01. These same objectively measured behaviors showed only weak associations with children's performance on assessments of cognitive regulation, median r = 0.11, p = 0.47. The findings indicate that ambulatory accelerometers can capture movement-based indicators of children's behavioral regulation in the classroom setting and that performance on measures of cognitive regulation does not strongly predict children's behavior in the classroom. As an unobtrusive and objective measure, actigraphy may become an important tool for studying children's behavioral regulation in everyday contexts.