Universal SEL interventions have garnered increasing attention in educational systems worldwide, including in Portuguese schools, with short- and long-term benefits for students, such as social and emotional competencies, academic performance, and positive attitudes and behaviours. Using a pre- and post-test design, the current study aims to analyse the effects of a universal SEL intervention on elementary school students' emotional problems, behaviour problems, hyperactivity behaviours, peer relationship problems, and pro-social behaviours, as well as the quality of peer relationships in the classroom. Participants included 207 fourth-grade students (50.5% females) from Portuguese public elementary schools, aged between 8 and 13 years (M[subscript age] = 9.15, SD = 0.56). Measures comprised the Strengths and Difficulties Questionnaire, completed by teachers, and the Classroom Peer Context Questionnaire, completed by students. Students attended a 32-week classroom-based universal SEL intervention during one school year. Results show a positive effect of the intervention on students' psychological adjustment (emotional symptoms, peer problems, hyperactivity), prosocial behaviours and mutual affection, even though small effect sizes were found. Male students reported higher peer problems and hyperactivity, which significantly improved at post intervention, while female students exhibited an increase in prosocial behaviours.