Sexually-transmitted infections (STIs) are a major public health problem. It has been established that they enhance the transmission of Human-Immunodeficiency Virus (HIV). The purpose of this study was to determine the level of knowledge possessed by high school learners about STIs. A quantitative, descriptive study was conducted at two high schools in the Blouberg municipality in the Capricorn district of Limpopo Province, South Africa. Data were collected from 298 randomly selected male and female learners aged 16 to 20 years, using a structured questionnaire. The mean score on the knowledge about the modes of transmission of STIs was 68%. Of the 67% who were sexually active, 68% started to have sex when they were between the ages of 14 and 16 years. Only 38% of the respondents recognised swelling of the lymph nodes on the groin as a sign of STIs. More than 60% knew that syphilis, gonorrhoea and HIV/AIDS were types of STIs, more than 50% knew genital herpes whereas less than 50% knew chlamydia trachomatis and the least known (by less than 40%) were Trachomonasvaginalis, Human Papilloma Virus, and Hepatitis B. Treatment of both partners was regarded by 56% as important whereas 44% regarded it as less important. High school learners be educated about lesser-known types of STIs and that treatment and prevention strategies, especially treatment of both partners, be emphasised in the education provided to these learners.Keywords: Sexually-transmitted infections, adolescents, knowledge.