Background: Recess and physical education time continue to diminish, creating a need for additional physical activity opportunities within the school environment. The use of school gardens as a teaching tool in elementary science and math classes has the potential to increase the proportion of time spent active throughout the school day. Methods: Teachers from 4 elementary schools agreed to teach 1 math or science lesson per week in the school garden. Student physical activity time was measured with ActiGraph GT3X accelerometers on 3 garden days and 3 no-garden days at each school. Direct observation was used to quantify the specific garden-related tasks during class. The proportion of time spent active and sedentary was compared on garden and no-garden days. Results: Seventy-four children wore accelerometers, and 75 were observed (86% participation). Children spent a significantly larger proportion of time active on garden days than no-garden days at 3 of the 4 schools. The proportion of time spent sedentary and active differed significantly across the 4 schools. Conclusions: Teaching lessons in the school garden may increase children’s physical activity and decrease sedentary time throughout the school day and may be a strategy to promote both health and learning.