Backgroundand Purpose: Outdoor play is widely acknowledged for its benefits to physical health and psychological well-being, yet its relationship with school readiness remains understudied in preschoolers. To address this gap in the literature, this study investigated how outdoor play relates to cognitive and psychosocial development among a nationally representative sample of preschoolers. Materials and methodData on the duration of outdoor play (on weekdays and weekends) and specific cognitive and psychosocial outcome variables (i.e., early learning skills, self-regulation, social-emotional development, and flourishing) were collected via questionnaires provided to the caregivers of preschoolers. Logistic regressions were performed to examine the associations between outdoor play with cognitive and psychosocial outcomes while adjusting for covariates and calculate the odds ratio with 95 % confidence intervals (CI). ResultsAmong the 10,682 preschoolers (i.e., 3-5y) included in this study (i.e., 5558 boys, Mage = 3.98 ± 0.99 years), there was a lower percentage of preschoolers engaged in outdoor play for over 3 h per day on weekdays (33.39 %) compared to weekends (56.85 %). Outdoor play in preschoolers is positively associated with various domains of school readiness, and engaging in outdoor play for more than 3 h per day is associated with more beneficial outcomes. ConclusionThe findings of this cross-sectional study suggest that promoting outdoor play among preschoolers can be an effective strategy for enhancing cognitive, social, and emotional development in this age group although further intervention studies are required to buttress this assumption empirically.