Abstract
Background/Objectives: Adolescence represents a period of heightened vulnerability to risky sexual behaviours. In Portugal, adolescents are initiating sexual activity at increasingly younger ages, making it relevant to characterise their knowledge about sexually transmitted diseases and prevention methods at an earlier stage of their development. Methods: A cross-sectional observational study was conducted involving the application of a questionnaire to 750 ninth-grade students from Vila Nova de Gaia, Porto, Portugal. The questionnaire covered topics such as perceptions of sexuality, knowledge of sexuality and STIs, methods of transmission prevention, and sources of information. Results: The mean age was 14.4 years, with 19.9% reporting having already initiated sexual activity. Overall, the knowledge about sexually transmitted infections was low, with a score of 7.08 out of a maximum score of 18. Condoms and immunisation were well-recognised preventive measures, but many misconceptions persisted. Better knowledge is associated with better attitudes regarding the likelihood of having sexual intercourse. Conclusions: This study reveals adolescents' limited knowledge about sexually transmitted infections, misconceptions about prevention, and reliance on peers and the internet over trusted sources, highlighting the need for comprehensive sexual education in middle school.