The study examined the influence of knowledge and awareness of sexually transmitted diseases on change in sexual behaviour of fresh undergraduates with a view to providing useful suggestions for positive sexual behaviour of adolescents. The study adopted the descriptive survey design. A sample of 600 fresh undergraduates was selected from the thirteen Faculties in O.A. U.; Ile-Ife, using convenience sampling technique. An adapted instrument titled "Questionnaire on Knowledge, Awareness of STDs and Sexual Behaviour" (QKASSB) was used to collect information from the respondents. The results showed that 93% of the fresh undergraduates had knowledge of the meaning of STDs, 88% of the respondents reported that they had awareness of STDs through media such as radio, television, internet etc. 88% of the respondents considered maturity as the first reason for change in fresh students' sexual behaviours. Finally, the result revealed a significant influence of sexual behaviour of fresh undergraduates before and after gaining admission into university (t-test = 9.039, p < 0.05). It was concluded that fresh undergraduates had better knowledge and got awareness of STD; But their exposure to new school environment had predisposed them to change in sexual behaviours.