The purpose of this research was to investigate reliable sources of sexual information for secondary school students that influence their sexual behaviour. This research was carried out in Kenya, specifically Thika West sub-County. It was conducted among sampled secondary school students, their head teachers and teacher counsellors. The specific objectives of the study were to: establish how sexual information obtained through the school curriculum influences secondary school students’ sexual behaviour; establish how sexual information received through the family setting influences secondary school students’ sexual behaviour; investigate how sexual information received through religious institutions influences secondary school students’ sexual behaviour; establish how sexual information obtained through the peer group influences secondary school students’ sexual behaviour; investigate how sexual information accessed through the media influences secondary school students’ sexual behaviour; and how students’ personal characteristics influence their sexual behaviour. The study was carried out using survey research design and was guided by six hypotheses. Research tools used comprised questionnaires for student respondents and interview schedules for head teachers and teacher counsellors. The data collected was processed using Statistical Package for Social Sciences (SPSS IBM statistics 21). The statistics were tabulated and subjected to regression analysis using ANOVA and coefficient models. Pertinent findings have indicated that: an overwhelming proportion of secondary school students are sexually active. This may have contributed in a rise in cases of premarital pregnancies and subsequent abortions, student drop out from schools, Human Immune Virus/Acquired Immune Deficiency Syndrome and Sexually Transmitted infections. The results established that mass media as a source of sexual information is the most influential, followed by peer group and school curriculum respectively.