Introduction: Sexual and reproductive health (SRH) education is crucial to prevent unwanted pregnancies and sexually transmitted infections (STIs) among young people. However, in Sri Lanka, little is known about the level of access to SRH education and knowledge among unmarried youth. This study aimed to assess perceptions of SRH education at school and the level of knowledge on selected aspects of SRH among unmarried youth aged 15–24 years in Sri Lanka. Methods: A cross-sectional study was conducted among 1057 unmarried male and female youth aged 15–24 years in three selected districts of Sri Lanka. A semi-structured, self-administered questionnaire was operationalized after receiving informed consent. Twelve focus group discussions were also conducted with the youth. Results: A sizable proportion of youth demonstrated low access to SRH education and a low level of knowledge on pregnancy, contraception, and STIs. Although it is limited, respondents felt that the SRH education received in school is useful for their life. Friends and the Internet were identified as the primary sources of information among boys, whereas the mother was the primary source for the majority of girls. Boys demonstrated better SRH knowledge compared to girls for most of the aspects. SRH knowledge was increased among both boys and girls significantly with maturity. Conclusion: Results conclude that there is a significant knowledge gap on SRH among unmarried youth. Introduction of age-specific and gender-sensitive SRH education is important to address the current gap in SRH knowledge.