Sub-Saharan Africa is more heavily affected by HIV/AIDS than any other region in the World. Half of all new HIV infections occur in young people. Identification of the associated factors is likely to be useful in designing effective interventions. This cross-sectional study aimed to investigate the determinants of high-risk sexual behaviours among youths in Kibaha District, Tanzania. Data was collected using a structured questionnaire. The survey gathered data pertaining to the sexual healthy behaviours among youths, including condom use, number of sexual partners, age at first sexual involvement and knowledge on sexually transmitted diseases and HIV/AIDS. A total of 322 individuals aged 15-24 years were involved in the study. More than 69% had sex at least once in their life time. Only about one-third (32.3%) of the youths reported to have used condom during the first sexual intercourse and 37% during the last sex. About 21.7% of the respondents acknowledged having more than one sexual partner in the last 12 months. The majority (98.4%) of the respondents have heard of HIV/AIDS. About three quarters (74.8%) of the respondents knew where to get HIV testing services but only a small proportion (28.9%) had tested for HIV infection. Of those not yet tested, 38.2% admitted that they were ready to do so. Although 317 (98.4%) respondents were aware of HIV/AIDS, and majority, 65.2% mentioned condom as the method used to prevent its transmission, only 117 (36.3%) acknowledged using them. In conclusion, despite good knowledge on transmission of HIV among youths in Kibaha district, only a small proportion of them practices safe sex. Education programmes on safe sex practices should be strengthened to provide skills that could be effective in changing and maintaining safe sex behaviours among youths in Tanzania.