The aim of this study was to describe some characteristics of vaginal, anal and oral sexual behavior in Spanish adolescents. It was a cross-sectional descriptive population study conducted using a probabilistic sample survey. The sample was composed of 4,612 male and female adolescents, of whom 1,686 reported having penetrative sexual experience. Sample size was established with a 97% confidence level and a 3% estimation error. Data collection took place in secondary education schools. Mean age of vaginal sex initiation was 15 years. Compared to females, males reported an earlier age of anal and oral sex initiation and a larger number of vaginal and anal sexual partners. Males also reported a higher frequency of penetrative sexual relations under the influence of alcohol or other drugs. A higher percentage of females than males reported not using a condom in their first anal sexual experience. This study provides a current overview of the sexual behavior of adolescents that can be useful for the design of future programs aimed at preventing HIV and sexually transmitted infections (STIs).