There are many factors which can influence sexual behavior in teenagers, and one of them is the source of information like social media. An interest in social media, the use of social media, and the intensity of using social media can influence teenagers' sexual behavior. Sexual behavior is all types of behavior which is caused by sexual desire, either with the opposite sex or with the same sex. Brown et.al (2006) points out that the exposure of social media which savors of pornography increases teenagers' sexual activity and expedites their desire to commit sexual intercourse among them. It seems that social media is considered as the source of sexual information. The objective of the research was to identify the influence of social media on teenagers' sexual behavior at SMA Swasta Prayatna, Medan, in 2015. The research was an analytic survey with cross sectional design. The population was 440 Grades X and XI students of SMA Swasta Prayatna, Medan, and 75 of them were used as the samples, taken by using systematic random sampling technique. The result of the research showed that 42.7% of teenagers' sexual behavior was in good category, and 57.5% was in bad category. There was the influence of the willingness to use social media (p = 0.002), the utilization of social media (p = 0.000), and the intensity of using social media (p = 0.007) on teenagers' sexual behavior at SMA Swasta Prayatna, Medan. The variable of which had the most dominant influence on teenagers' sexual behavior was the utilization of social media at coefficient β = 25.176. It is recommended that the students use social media in positive sense in which the use of social media as the correct information service based Web. The management of the school should increase supervision and encourage young generation to use social media in the positive sense and anticipate the change in behavior and moral degradation.