OBJECTIVE: To describe the sexual behavior of freshmen undergraduate students according to demographic, economic, psychosocial and behavioral characteristics, and evaluate the prevalence of risky sexual behavior and its associated factors. METHODS: A cross-sectional study of the census type with undergraduate students over 18 years old of 80 undergraduate courses of the Universidade Federal de Pelotas (UFPel), in Rio Grande do Sul (RS), who entered in the first semester of 2017 and remained enrolled in the second semester. Undergraduate students who reported having had sex were evaluated. We considered as risky sexual behavior having more than one sexual partner within the last three months and not having used condoms in the last sexual intercourse. RESULTS: The prevalence of risky sexual behavior was 9% (95%CI 7.6–10.5). Men presented more risky behavior than women, with a prevalence of 10.8% and 7.5%, respectively. Of the undergraduate students, 45% did not use condoms in the last sexual intercourse, and 24% had two partners or more within three months before the survey. Smartphone applications for sexual purposes were used by 23% of students within three months before the survey. Risky sexual behavior was associated with gender, age at first sexual intercourse, frequency of alcohol consumption, consumption of psychoactive substances before the last sexual intercourse and use of smartphone applications for sexual purposes. CONCLUSION: Although undergraduate students are expected to be an informed population, the prevalence of risky sexual behavior was important, indicating the need to expand public investment in sexual education and awareness actions.