Background: Body image issues are prevalent among adolescents and associated with negative physical and mental health outcomes. Despite a growing body of literature to indicate social media as an appearance pressure, at the time of commencing the current research (2017) there was no evidence worldwide of an intervention to promote positive body image and social media literacy. Since then, while a few interventions have been piloted and evaluated, many aspects of the current intervention remain novel, particularly with respect to the use of a co-design development approach and inclusion of a qualitative component to evaluate programme acceptability. Research Objectives: The primary objectives of this mixed method, multi-stage research were to: (1) explore the narratives and experiences related to body image in a sample of Irish adolescents; (2) co-design, in collaboration with adolescents, teachers, and decision makers, a school-based intervention to promote positive body image and social media literacy in adolescents (#More Than A Selfie (#MTAS)); and (3) pilot test the #MTAS intervention in a sample of young male and female adolescents (aged 11-14 years) to assess its initial effectiveness to improve body image, social media literacy and promote a supportive peer environment. A secondary objective of the study was to develop resources as a means of increasing awareness of body image issues more broadly, both in schools and in the wider community. Method: Stage One involved the use of co-creation/participatory approaches in the development of the #MTAS intervention including focus groups, interviews, and co-creation workshops. Stage Two entailed two phases including: (1) a non-randomised pilot evaluation of the intervention involving 161 adolescents (84 males, 77 females), teachers (3) from three schools; and (2) a mixed methods assessment of adolescent and teacher participants’ experiences and views of the #MTAS (via; feedback questionnaires with open-ended questions; focus group). Stage Three focused on the collaborative development of a suite of resources aimed at translating the findings into practice, whilst also promoting awareness of body image issues more generally. Results: A four-week intervention (#MTAS) was successfully developed through an extensive co-creation process. The pilot evaluation demonstrated statistically significant improvements (p < 0.05) in the intervention versus control group on attitudes to body image, time spent on social media and perceived social support in males, as well as improved social media literacy and reduction in perceived appearance pressure from family in females. The experiences and views of the intervention (including delivery) were very positive. The programme was considered highly acceptable by teachers and students with 91% of students stating, ‘it was good for their class’ and 88% indicating that they would ‘recommend the #MTAS to a friend’. Conclusion: This study is the first to utilise an iterative process of active and meaningful co-creation (with numerous stakeholder groups) in the development of a body image and social media literacy intervention. The inclusion of adolescent stakeholders across the research life cycle represents an important addition to the very limited literature in this area. The collective findings, despite some study limitations, provide promising initial evidence on the effectiveness and feasibility of the new co-designed, teacher-delivered, #MTAS programme for use in Irish schools as part of the SPHE curriculum. Following careful consideration and detailed discussion (with stakeholders and the research team) a new website was developed to disseminate the programme and support its delivery and translation into practice. Several avenues for future research are indicated.