Suicidal behaviour is a major socio-health problem worldwide. However, there are few empirically validated programs for universal prevention of suicidal behaviour in school settings. The aim of the present study was to design and validate the "PositivaMente" program for the prevention of suicidal behaviour in school-age adolescents aged 14-15 from the North of Spain. A quasi-experimental design was used with pre- and post-treatment evaluation with experimental and control groups and a six-month follow-up. The final sample consisted of 264 participants (M = 14.30 years, SD = 0.56; 54.5% girls), with 161 participants in the experimental group and 103 in the control group. Measuring instruments were administered to assess suicidal behaviour, emotional and behavioural difficulties, depressive symptomatology, prosocial behaviour, subjective well-being, and self-esteem. The "PositivaMente" program was designed and implemented in educational settings. A statistically significant improvement in subjective well-being was found, as well as a statistically significant reduction in emotional problems and problems with peers among female participants in the experimental group versus those in the control group at the 6-month follow-up. However, male participants did not seem to benefit from the program. The overall evaluation from the sample and satisfaction with "PositivaMente" were positive. Empirically supported actions for the prevention of suicidal behaviour need to be designed in order to make informed decisions. Future studies should implement the "PositivaMente" program with other populations and contexts, develop a brief version, and collect information on cost-effectiveness.