Background: Improved access to sexual and reproductive health (SRH) services and information is essential for supporting adolescents and youth in making informed decisions and optimizing each young person's outcomes related to their SRH, health and well-being and countries' current and future social and economic development. Mobile phones offer opportunities for young people to privately access SRH content and to be linked to SRH services. The objective of this study was to develop the content for an SMS (short message service or "text message") platform jointly with adolescents and youth in three regions in Peru (Lima, Ayacucho and Loreto) as part of the ARMADILLO (Adolescent/Youth Reproductive Mobile Access and Delivery Initiative for Love and Life Outcomes) Study. Methods: Content development was done in three stages. During Stage 1, we held community consultations with 13-17 year old adolescents, 18-24 year old youth and professionals who work with young people through the education and health sectors ("adult advisers") to identify and rate SRH topics of interest through group free- and guided-brainstorming activities and an individual written sharing activity. During Stage 2, the team developed the preliminary domains, sub-domains and content for the SMS platform. During Stage 3, we held focus groups with adolescents to validate the SMS content, including both individual scoring of and group feedback for each SMS. Group feedback asked about their general impressions and understanding and their thoughts about the language and usefulness of the SMS. Results: A total of 172 adolescents and youth ages 13-24 and 20 adult advisers participated. Adolescents and youth brainstormed and rated SRH topics and sub-topics that led to the initial structure for the SMS platform, with 9 domains, 25 sub-domains and 146 draft SMS. Adolescents provided high scores for the SMS, with all sub-domains receiving average scores of 3.0 or higher (out of 4.0) for the SMS included. Adolescents also provided suggestions to optimize content, including improvements to unclear messages, resulting in SMS with adolescent-friendly content in simple, straightforward language. This process also revealed that adolescents lacked knowledge and had misconceptions related to contraceptive methods. Conclusion: This study details the systematic process used to develop relevant and accessible SRH information through a participatory approach. We document critical information about what young people know and how they think, enabling us to understand their perspective and literally speak their language. Results also provide future directions for programmatic, research and policy efforts with young people, in particular around gender norms, interpersonal violence, and access to SRH information and services, in similar settings.