Unhealthy sexual behavior among adolescents, especially unmarried adolescents, tends to increase. This is evident from a survey in several developed countries which show that the United States has a pregnancy rate (age 15-19 years) of 95/1000. Adolescence is a transitional period marked by physical, emotional and psychological changes. Premarital sexuality in adolescents is sexual relations performed by adolescents before marriage. This study aims to determine the relationship between knowledge and attitudes about premarital sexuality at SMK YPK Medan. The design of this study was a correlational analytic cross sectional approach with a total sample of 127 people using the stratified random sampling method. The instrument in this study was a questionnaire which included demographic data and a pre-marital sexual questionnaire. The results showed that most adolescents with good knowledge were 63 people (49.60%) and negative as many as 89 people (70.07%). After the chi-square test was carried out with a confidence level of 95% or α = 0.05, it was found that the value of X2 = 49.273> X2 table (5.991), the value of p = 0.000, which means that it can be concluded that there is a significant relationship between adolescent knowledge and attitudes about sexuality premarital.