HIV is a growing public health problem in Europe, with men-having-sex-with-men and migrants from endemic regions as the most affected key populations. More evidence on effective behavioral interventions to reduce sexual risk is needed. This article describes the systematic development of a theory-guided computer-assisted safer sex intervention, aiming at supporting people living with HIV in sexual risk reduction. We applied the Intervention Mapping (IM) protocol to develop this counseling intervention in the framework of a European multicenter study. We conducted a needs assessment guided by the information-motivation-behavioral (IMB) skills model, formulated change objectives and selected theory-based methods and practical strategies, i.e. interactive computer-assisted modules as supporting tools for provider-delivered counseling. Theoretical foundations were the IMB skills model, social cognitive theory and the transtheoretical model, complemented by dual process models of affective decision making to account for the specifics of sexual behavior. The counseling approach for delivering three individual sessions was tailored to participants' needs and contexts, adopting elements of motivational interviewing and cognitive-behavioral therapy. We implemented and evaluated the intervention using a randomized controlled trial combined with a process evaluation. IM provided a useful framework for developing a coherent intervention for heterogeneous target groups, which was feasible and effective across the culturally diverse settings. This article responds to the need for transparent descriptions of the development and content of evidence-based behavior change interventions as potential pillars of effective combination prevention strategies.