This article describes the development of ProjectHeartforGirls.com, an interactive web-based program designed to improve sexual communication skills and reduce the risk of HIV/STDs among adolescent girls, a population at heightened risk for negative sexual health outcomes (Centers for Disease Control and Prevention, 2013). Although sexual communication is a critical predictor of safer sex among teens, there are few online interventions that target these skills as a central program component. We developed ProjectHeartforGirls.com to fill this gap. Program development involved (1) identifying the target population (ethnically diverse high school girls), (2) clarifying the theoretical foundation (Reasoned Action Model), (3) conducting formative qualitative research (n = 25 girls), (4) drafting initial program content, (5) receiving ongoing feedback from a teen advisory board (n = 5 girls), (6) programming online content, and (7) conducting usability testing (n = 6 girls). These steps are described along with the final intervention product, which is currently being evaluated in a randomized controlled trial.