To evaluate the effect of an internet-based educational intervention to increase knowledge of sexually transmitted infections (STIs), attitudes and self-efficacy toward consistent condom use in Mexican adolescents. A field trial with an intervention and control group was conducted in 14- to 15-year-old students in two secondary schools. The intervention was delivered via a website that included four educational sessions during a 4-week period and six 30-min class discussions during a 3-month period. In the control group, the investigators observed the general sex education provided by the school. Outcome variables were (1) knowledge about STIs, (2) attitudes regarding condom use, and (3) self-efficacy toward consistent condom use. Differences-in-differences (Diff-in-Diff) treatment effect was estimated for each outcome variable. There were 246 adolescents in the intervention group and 210 in the control group. The intervention had a positive effect on improving knowledge of STIs, attitudes and self-efficacy toward consistent condom use. The major effect was observed on adolescents' knowledge on STIs (Diff-in-Diff 30.34 points, P < 0.0001). A youth-friendly, culturally-contextualized, internet-based educational intervention complemented by class discussions may be a significant addition to the regular secondary school sex education program to improve knowledge of STIs, attitudes and self-efficacy toward consistent condom use among adolescents.