<p>Outdoor learning is widely assumed to potentiate social–emotional learning (SEL) in early childhood, yet rigorous cross-national randomized evidence remains limited. To address this gap, we conducted a multi-site, parallel-group, three-arm randomized trial in public preschools in Slovakia and Spain evaluating a 20-week nature-oriented SEL curriculum delivered predominantly indoors (≤ 1&#xa0;h outdoors weekly) or enacted through planned outdoor learning (≥ 4&#xa0;h/day outdoors) against routine pedagogy (control). The study included <i>N</i> = 272 participants: a child cohort (<i>n</i> = 118) randomized within country to indoor-SEL, outdoor-SEL, or control, and adult cohorts of teachers (<i>n</i> = 18) and parents (<i>n</i> = 136). The primary outcome was perceived prosocial behavior independently rated by teachers and parents using the Questionnaire to Assess Perceived Prosocial Behavior in Preschool Children (QSPBPCH) at pretest and posttest. Primary analyses used Group × Time mixed-design ANOVAs with Bonferroni-adjusted contrasts, complemented by linear mixed-effects models accounting for classroom clustering and country. Relative to control, both SEL arms showed greater pretest–posttest gains across rater groups, whereas the contrast between outdoor and indoor delivery formats was not statistically significant at posttest. A nature-oriented SEL curriculum appears feasible within public preschool systems and—whether enacted through outdoor learning or indoors—is associated with improvements in perceived prosocial behavior. The absence of a statistically significant contrast between delivery formats highlights methodological challenges in evaluating SEL interventions in early childhood contexts and underscores the need for robust and transferable measurement approaches capable of capturing contextual influences across educational settings.</p>