Background: Little is known about the influence of Montessori methods on children's physical activity (PA). This cross-sectional study compared PA of children attending Montessori and traditional preschools. Methods: We enrolled 301 children in 9 Montessori and 8 traditional preschools in Columbia, South Carolina. PA was measured by accelerometry on weekdays during preschool (In-School), non-school (Non-School), and all day (All Day). Minutes/hour of light, moderate-to-vigorous (MVPA), and total PA (light?+?MVPA) were calculated. Results: Children attending Montessori preschools accumulated more In-School light (7.7 vs. 6.5?minute/hour), MVPA (7.7 versus 6.5?minute/hour), and total PA (15.4 versus 13.0?minute/hour) than children attending traditional preschools, after adjusting for sex, race/ethnicity, body mass index, parent education and neighborhood poverty index. For Non-School (8.5 versus 6.2?minute/hour) and All Day (8.5 versus 7.6?minute/hour), children in Montessori preschools accumulated more MVPA than children in traditional preschools. In-School PA was higher for children in private Montessori than public Montessori preschools (8.1 versus 7.0?minute/hour; 8.1 versus 6.7?minute/hour; 16.1 versus 13.6?minute/hour, for light, MVPA, and total PA, respectively). Conclusions: Children attending Montessori preschools were more active than children attending traditional preschools. Adopting the Montessori system may be an important strategy for promoting PA in children.