This study examined the concurrent and long-term relations between the amount of time children attending daycare spend outdoors and their cognitive and behavioral development during preschool and first grade. We applied a multi-informant design using cognitive testing and ratings from parents and teachers to follow 562 Norwegian preschoolers (298 girls; mean age at first assessment = 52.45 months, SD = 15.96) over a period of four years. Children’s attention skills were tested with the digit span test while teachers rated their behavior. Growth curve analyses showed a positive relation between outdoor hours and children’s digit span scores, and an inverse relation between outdoor hours and inattention-hyperactivity symptoms, controlling for a host of possible confounds. Results indicate that outdoor time in preschool may support children’s development of attention skills and protect against inattention-hyperactivity symptoms.