Introduction Adolescents are more vulnerable than adults on sexually transmitted diseases and HIV/AIDS. Among the adolescents, girls are more vulnerable to STDs including HIV/AIDS. Their knowledge about STDs and HIV/AIDS is very poor. This study was conducted between February 2016 and September 2016 investigating adolescent’s between 10–19 years knowledge about sexually transmitted diseases including HIV/AIDS, its mode of transmission and prevention methods.

Method A multistage cluster sampling technique was used to select the sample. Data on 300 upper primary school female adolescents and 300 high school female adolescents was analysed. The study found that a large proportion of upper primary school adolescents ages between10-14 had very little knowledge on sexually transmitted diseases and HIV. More than three quarter of high school adolescents ages 15–19 years had knowledge about HIV respectively. On an average, about one third of them had no knowledge on HIV in terms of modes of transmission and prevention.

Results The multivariate logistic regression analysis revealed that adolescent age, years of schooling and knowledge on STIs and STDs appeared to be important predictors of the awareness about HIV/AIDS.

Conclusion Useful and fruitful awareness campaigns to educate the upper primary female adolescents regarding the health consequences of STDs including HIV/AIDS and integrated approach is strongly suggested for creating knowledge and awareness to control the spread of HIV/AIDS among adolescents in Ndonyo Sabuk, Murang’a County, Kenya.