Tanzania is realizing an increase in adolescents engaged in multiple sex partner behaviour and premarital sex. The objective of this study was to assess sexual behaviour and risk factors among secondary school students in Moshi rural district in Kilimanjaro, Tanzania. Anonymous questionnaires were completed by 360 students in six secondary schools randomly selected by gender and students' grade. Statistical analyses were performed using descriptive statistics. Of 360 students, 55.3% reported ever having sexual intercourse, and 18.1% reported having multiple sex partners. Of all students who had engaged in sexual intercourse, 59.6% reported having a single sexual partner and 40.4% of them reported having multiple partners. Several demographic, family, peer and work influences, and student factors including knowledge and attitude toward sex were risk factors for students having sex. About 60% of the sexually active students in both sexes reported to have never used condoms in the previous year. Both pregnancy and induced abortion had been experienced by about 12% to 27% of the sexually active female students and about 10% of the sexually active students of both sexes reported to have been diagnosed with Sexually Transmitted Diseases (STDs). The results also show that students were likely to practice masturbation, start having sex at a younger age, and have sex with married men/women and/or men/women not their "sexual partner" at first coitus. With the risks of STDs and Human Immunodeficiency Virus (HIV) among young people, teaching sex education at secondary schools in Tanzania offers the best hope of raising awareness in students who are approaching sexual maturity.