BACKGROUND: Early adolescent unintended pregnancy and sexually transmitted infection prevention are significant public health challenges in the United States. Parental influence can help adolescents make responsible and informed sexual health decisions toward delayed sexual debut; yet parents often feel ill equipped to communicate about sex-related topics. Intergenerational games offer a potential strategy to provide life skills training to young adolescents (aged 11-14 years) while engaging them and their parents in communication about sexual health. OBJECTIVE: This study aims to describe the development of a web-based online sexual health intergenerational adventure game, the Secret of Seven Stones (SSS), using an intervention mapping (IM) approach for developing theory- and evidence-based interventions. METHODS: We followed the IM development steps to describe a theoretical and empirical model for young adolescent sexual health behavior, define target behaviors and change objectives, identify theory-based methods and practical applications to inform design and function, develop and test a prototype of 2 game levels to assess feasibility before developing the complete 18-level game, draft an implementation plan that includes a commercial dissemination strategy, and draft an evaluation plan including a study design for a randomized controlled trial efficacy trial of SSS. RESULTS: SSS comprised an adventure game for young adolescent skills training delivered via a desktop computer, a text-based notification system to provide progress updates for parents and cues to initiate dialogue with their 11- to 14-year-old child, and a website for parent skills training and progress monitoring. Formative prototype testing demonstrated feasibility for in-home use and positive usability ratings. CONCLUSIONS: The SSS intergenerational game provides a unique addition to the limited cadre of home-based programs that facilitate parent involvement in influencing young adolescent behaviors and reducing adolescent sexual risk taking. The IM framework provided a logical and thorough approach to development and testing, attentive to the need for theoretical and empirical foundations in serious games for health.