Objective: The high prevalence of sexually transmitted infections (STIs) including human immunodeficiency virus (HIV) among Nigerian young people has been linked to poor knowledge, attitudes and risk behaviour. This study evaluated the effectiveness of a sexuality education programme (SEP) in improving adolescents' STI/HIV knowledge and attitudes and reducing risk behaviour. Design: A quasi-experimental pre-post study was conducted. The experimental group (n = 189) and comparison group (n = 195) comprised senior secondary school students. Data were collected via a validated self-report questionnaire. A paired "t"-test, an independent "t"-test and a one-way between-group analysis of covariance (ANCOVA) were used to analyse the data. Setting: Three hundred eighty-four young people aged 15-20 years at low and high risk for STI/HIV infection participated in the SEP in a school setting in Northeast Nigeria. Results: There were significant post-intervention improvements in students' knowledge, attitudes and practices in the experimental group (p < 0.001). A "t"-test showed that after the SEP, the mean scores of STI/HIV knowledge (11.25 ± 3.39 vs 8.23 ± 3.60) and attitudes (29.63 ± 6.21 vs 35.38 ± 7.86) were significantly higher in the experimental group than that in the control group. Similarly, the mean risk behaviour score (6.83 ± 2.10 vs 5.64 ± 2.27) was higher in the experimental group than that in the control group post-intervention. Conclusion: The school-based SEP increases STI/HIV knowledge, improves positive attitudes and reduces STI/HIV risk behaviour among school young people in Nigeria.