This paper presents findings of a feasibility study to adapt and evaluate the impact of an evidence-based HIV prevention intervention on sexual risk behaviors of in-school 6th grade youth in post-conflict Liberia (n = 812). The study used an attention-matched, group randomized controlled trial. Four matched pairs of elementary/middle schools in Monrovia, Liberia, were randomly assigned to either an adapted eight-module HIV prevention or a general health curriculum. Three- and nine-month impacts of the intervention on sexual risk behaviors and on mediating variables are presented. The intervention significantly impacted protective peer norms and positive condom attitudes and increased frequency of condom use at the nine-month follow-up. The intervention did not impact sexual initiation or multiple sex partnerships. Future intervention research should address the salient pressures that are unique to post-conflict settings and include longer follow-up time periods and smaller class sizes to meaningfully impact sexual initiation and multiple sex partnerships.