Objective To evaluate the level of knowledge and preventive measures used against Sexually Transmitted Infections (STIs) in Andean adolescents. Methods Quantitative, descriptive, cross-sectional study, with 200 school adolescents from the Andean districts of the Junín region-Peru. Knowledge of concepts, forms of transmission and preventive measures before STIs was recorded through a questionnaire validated by expert judgment in a pilot test. Results 82% of adolescents answered incorrectly about the concept of STIs, but 89% responded correctly about the definition of AIDS. More than 85% know that sexual intercourse is the main form of STI transmission and that sexual intercourse, blood transfusions and mother-to-child transmission are the main forms of HIV transmission; however, 70% of adolescents did not respond correctly about the correct sequence of condom use. Conclusion The level of knowledge of Andean adolescents in Peru regarding sexually transmitted infections is intermediate to low; Therefore, it is necessary to strengthen public policies for the prevention of STIs in this vulnerable population.