In this study, we aimed at comparing Finnish primary school students' physical activity and sedentary time during outdoor-learning and traditional school days. Outdoor learning is defined as purposeful and planned learning experiences in the outdoors . The study population consisted 20 students in grades 1-6 at one primary school. Physical activity was measured objectively by hip-worn accelerometers. The amounts of physical activity and sedentary time were compared for the same students between five school days with outdoor learning and four traditional school days. The study's main findings show that, moving from traditional classroom teaching to an outdoor-learning environment decreases students' sedentary time and increases light- and moderate-intensity physical activity during the school day. No differences were observed in leisure-time physical activity on school days with or without outdoor learning. Thus, outdoor learning is an effective complement to traditional classroom teaching in promoting physically active children.