Key Social Norms Take-aways This study highlights how social media-based health education interventions can positively shape adolescents perceived social norms related to sexual health behaviors. Although perceptions of other’s feelings towards a behavior (coined as subjective norms) were not the strongest predictors of behavioral intention, the intervention fostered a greater sense of peer approval and support for sexually transmitted infection prevention, contributing to improved attitudes and intentions around safe sexual practices. Objectives: The study aimed to assess the effectiveness of a social media-based health education intervention in influencing adolescents’ intentions to engage in safer sexual health behaviors in the Western Terai region of Nepal. Methods: Using a quasi-experimental design, 160 adolescents aged 14–19 from four schools were divided into intervention and control groups. Data were collected through pre- and post-intervention self-administered questionnaires based on the Theory of Planned Behavior, measuring knowledge, attitudes, subjective norms, perceived behavioral control, and intentions. Statistical analysis included paired and independent t-tests, correlation, and multiple regression. Intervention Details: The intervention was delivered over 15 days via Facebook Messenger and included infographics, quizzes, e-posters, and interactive discussions on sexual health and sexually transmitted infection prevention. Materials were culturally tailored and validated through expert and adolescent input to ensure relevance and clarity. Active engagement was encouraged through knowledge quizzes and discussion prompts. Results: The intervention significantly improved knowledge, attitudes, subjective norms, perceived behavioral control, and behavioral intentions compared to the control group. While attitude and perceived behavioral control were significant predictors of intention, subjective norms did not significantly predict intention but showed improved posttest scores. Participants reported increased sexually transmitted infection awareness, better communication with partners, and debunking of sexual health myths. Conclusions: Social media platforms like Facebook Messenger offer an effective and cost-efficient channel for delivering sexual health education to adolescents. The intervention successfully improved critical psychosocial factors influencing sexually transmitted infection prevention behaviors. Importantly, while attitude and self-efficacy were key drivers of intention, enhancing perceived social norms remains a valuable component for comprehensive behavior change strategies targeting youth sexual health.