Abstract: The study investigated the relationship between the sexual behaviour and knowledge of HIV/AIDS among the youths in South West Nigeria. The study also investigated the different types of sexual behaviour and whether youths have the knowledge of key basic concept on HIV/AIDS. Using the descriptive research design, a total number of 1,420 undergraduates who correctly filled the self constructed questionnaire in 4 different universities in 4 different states of South West geographical zone of Nigeria were sampled. A self constructed questionnaire which was validated using face, content and construct method of validation was used. The instrument had a construct and a test retest reliability of 0.76 and 0.85, respectively. Pearson Product Moment Correlation and t-test were used in the inferential analysis, while frequency counts and percentages were used in describing the knowledge and sexual behaviour of youths. The result of the analysis revealed that most respondents were sexually active and were engaged in high risk sex such as casual, same sex, multiple sex and sex in exchange for money or favour. It was clear that youths have very high knowledge of key basic concept on HIV/AIDS but many youths have misconceptions about the cure of AIDS. The study recommended that policies and programmes that can transform the sexual life of youths or reduce their risk behaviours should be put in place while prevention message should be consistent, clear and effective to counteract other unreliable sources of information.