Key Social Norms Take-Aways This study shows that participatory, values-based education can shift deeply held social norms among adolescents. By encouraging open dialogue and reflection on stigmatized issues like abortion and contraception, the intervention helped reduce judgment and promoted supportive attitudes. Addressing these interconnected stigmas is critical for enabling young people to make informed reproductive decisions and for fostering environments where adolescent agency and equity in health can thrive. Objectives: This study evaluated the effectiveness of a school-based intervention aimed at reducing stigma related to abortion and contraceptive use among adolescents in Kisumu County, Kenya. It examined whether attitudes and beliefs could be shifted through values-based, participatory education within a secondary school setting. Methods: A 12-month quasi-experimental, controlled before–after study was conducted in two mixed-gender secondary schools (n=1368) in peri-urban Kisumu. Students completed surveys at baseline, 1-month, and 12-month follow-up. The 18-item Adolescents Stigmatizing Attitudes, Beliefs and Actions (ASABA) scale measured abortion stigma, and the 7-item Contraceptive Use Stigma (CUS) scale assessed stigma toward contraception. Statistical analyses included paired-sample t-tests and repeated measures ANOVA to evaluate changes in mean scores over time. Intervention Details: The intervention school received a 4-session, 8–9 hour stigma-reduction program grounded in the IPPF’s How to Educate about Abortion guide. Sessions used interactive methods—role-plays, discussions, Q&As, and values clarification exercises—to address abortion and contraceptive stigma and promote gender-equitable attitudes. The program was developed in consultation with students, teachers, health professionals, and community stakeholders. The control school continued with Kenya’s standard comprehensive sexuality education (CSE). Results: After 12 months, abortion stigma (ASABA) scores in the intervention school decreased by 30.1% and contraceptive use stigma (CUS) by 27.3%. In contrast, the control school saw only modest reductions of 9.0% and 7.9%, respectively. Boys initially showed higher stigma scores but experienced greater reductions than girls over time. A strong positive correlation between ASABA and CUS scores suggested that abortion and contraceptive stigma are closely related. Conclusions: The stigma-reduction intervention significantly improved adolescents’ attitudes toward abortion and contraception compared to standard CSE. Integrating such content into school-based sexuality education can more effectively challenge harmful beliefs and support gender-equitable reproductive health.