Increased rates of sexually transmitted diseases (STDs) among young adults between the ages of 15 and 24 years old had been one of the major concerns of the world. For each of the 20 million new recorded STDs cases, people within those age group were responsible for more than 50% of them. This cross-sectional study was done to determine the knowledge, attitudes, risky behaviours and preventive practices related to sexually-transmitted diseases (STDs) among undergraduate students in a public university in Malaysia. A total of 1327 university students aged between 19 and 27 years were surveyed by using a self-administered questionnaire. The majority (75.1%) had good knowledge on STDs, and a significant number of students (53.8%) had acceptable preventive practice. Marital status was associated not only to the students’ level of knowledge but also with their preventive practices. Despite having a good knowledge of STDs, students in Science and Technology academic cluster perceived unacceptable preventive practices. This study highlighted some misconceptions about STDs, preventive practices and risky behaviours, raising concerns about a likely future rise in STD incidence. Therefore, interventions to implement strengthened health education on STD-related issues were required to bring change in practices, along with knowledge and attitudes.