Risky sexual behaviors have high personal, social, and financial costs. Fathers’ talk with teens about sex can protect teens from risky sexual behavior. However, the positive effects are limited, as few fathers talk with their teens about sex and those who do talk report low frequency of talk. Impacts of parent programs to support teen reproductive health are reduced by low participation and retention and lack of attention to father-specific barriers and supports for talk with teens about sex. The proposed intervention will address limitations of existing programs by using an online design with self-paced lessons and engaging both fathers and teens, and add innovative elements identified in father-based needs assessments such as a father-facilitated peer-based support group and identifying multiple approaches to talk with teens. Using Intervention Mapping, a systematic protocol using theory, empirical evidence, and community input to develop health interventions, we will develop an online intervention to support fathers’ health-promoting talk with their teens about sex and relationships, using Social Cognitive Theory, and evaluate the intervention’s acceptability, feasibility, and preliminary efficacy. The short-term goals of the intervention are to increase health-related knowledge, self-efficacy, and outcome expectations as mechanisms to reduce teens’ sexual risk behavior. We plan to use a future R01 to conduct a full-scale randomized control trial to assess the program’s efficacy to address long-term goals identified by NIH of reducing teens’ STI transmission and increasing their contraceptive use. The aims of the current study are 1) To develop a 3-module prototype for a father-teen online program to support health-promoting family communication about sex and relationships, and 2) To evaluate the program’s acceptability, feasibility, and preliminary efficacy. To address Aim 1, we will develop program modules using online lessons and interactive activities for fathers and teens to complete together, with ongoing stakeholder feedback from our Family Advisory Board, which includes 10 fathers and ≈6 teens. Aim 2 will assess the program’s acceptability, feasibility, and preliminary efficacy using a pilot study of 50 pairs of high school-aged teens and fathers. Acceptability and feasibility assessment use data such as pilot enrollment, participant feedback, and survey questionnaires. Given the limited project scope, preliminary efficacy will be explored by assessing initial directionality for correlates of teens’ sexual behavior through pre- and post-intervention surveys, using matched pair t-tests and t-tests to assess effects of teen gender on outcomes. This study is significant as it translates research on father-teen talk about sex into a technology-based and potentially scalable intervention designed to reduce teens’ sexual risk behavior. This study’s innovation lies in developing one of the first interventions to address fathers as an under-u...