Studies focusing on the relevance or impact of a program, not just on its efficacy or effectiveness, can make important contributions to prevention science. This article documents the social validity (i.e., a construct encompassing feasibility, acceptability, and utility) of a universal substance use prevention program for early adolescents in Spain. The Mantente REAL (keepin’it REAL) program was culturally adapted to the Spanish context, implemented, and evaluated in six public middle schools in two regions of Spain. Participating teachers (N = 15), students (N = 354), and research team members (N = 6) reported on the feasibility, acceptability, and utility of the program implemented with first grade secondary school students. Qualitative and quantitative data about the program, its curriculum, and the implementation process were collected through teachers’ focus groups, students’ surveys, and observation forms completed by members of the research team. Mantente REAL was perceived to be a prevention program that was feasible for implementation in Spanish middle schools, although some logistics related to school structural constraints should be addressed in future implementations. The topics and activities in the curriculum were highly accepted by teachers and students, and they reported that the program was useful in teaching resistance strategies to cope with substance use and other risky situations. The findings support the social validity of the culturally adapted Mantente REAL program for early adolescents in Spain, and highlight how feedback from stakeholders involved in the implementation can improve the dissemination of effective prevention approaches.