This study evaluated the effect on reported readiness for action and attitudes toward obesity prevention among older adolescents (mean age 17) who took part in a youth-led participatory action research European initiative (CO-CREATE Youth Alliances) compared with a comparison group that acted as controls. This was a concurrent before-and-after controlled study across five countries and took place between September 2019 and October 2020. Adolescents (n = 159) recruited from schools and youth organizations came together with researchers and formed 15 Youth Alliances. An online questionnaire measuring their readiness for action and attitudes toward obesity prevention was administered. Alliance members (n = 62) who filled in the questionnaire at both baseline and postinitiative, and adolescents from the comparison group (n = 132) who completed the questionnaire twice were included in the main analysis. Two-level linear mixed models controlling for country-related variance were fitted. Alliance members scored significantly higher than the comparison group on two factors in each of the readiness for action, responsibility, and drivers of behavior concepts. The findings suggest that involving youth in co-creating policies to prevent obesity may increase adolescents' readiness for action and promote a shift in adolescents' conceptualization of obesity from an individual perspective to a societal responsibility and drivers of behavior.