Mental health during childhood is a growing social challenge with important implications for optimal development. Social and Emotional Learning (SEL) interventions have been shown to prevent problems related to psychological maladjustment, as well as promote emotional well-being. This article aims to provide evidence for the impact of RULER (Recognizing, Understanding, Labeling, Expressing, Regulating), an evidence-based approach to SEL, on students' mental health outcomes in Spanish public schools. A total of 207 primary school students (50.24% girls, mean age of 9 years) participated in the study. The teachers in the intervention school as compared to the control school received RULER instruction consisting of 15 hours of professional development training, 3 hours of follow-up, and SEL implementation guidelines throughout the school year. Students completed the Behavior Assessment System (BASC S-2). Results showed significant impacts in the RULER as compared to control school, including a reduction of clinical symptoms, such as anxiety and atypicality, as well as emotional symptoms, such as the sense of inadequacy among 9 to 12 year old students. Implications of these findings are discussed.