This article describes STI and HIV transmission knowledge and risk perception for undocumented Nicaraguan labor migrant women in semirural Costa Rica. Within all of Latin America, HIV prevalence is highest in the subregion of Central America; in rural areas it is increasing among heterosexual women. The mixed methods study included semistructured questionnaires administered in face-to-face interviews with 43 undocumented Nicaraguan migrant women in a three-interview series. Questionnaire items included sociodemographic variables and general knowledge about STIs and HIV modes’ of transmission and sources of education. Risk vignettes and follow-up questions were administered to assess culturally available prevention strategies as well as STI and HIV risk perception. As a mode of prevention, condom use is not culturally available to this population because of symbolic meanings behind requests to use them. Ethnographic results revealed that condom use is not culturally available to heterosexual women migrants in marital unions, there is a general knowledge of modes of transmission and prevention, yet communication between partners could be improved. Future interventions must address cultural concepts that shape risk perception as well as social risks perceived in the prevention of STI and HIV transmission.