This study determined the sources from which Vietnamese high school students currently acquire sexual knowledge as well as the sources from which they believe they should acquire it. Participants were eleventh-grade students at three public high schools in Hanoi, Vietnam. We conducted a questionnaire-based survey from September to October 2012, with 1672 students participating. Responses were received from all 1672 students, a response rate of 100%. Complete data were received from 1653 respondents (98.9%), including 896 males (54.2%) and 757 females (45.8%). The survey was a self-administrated questionnaire. Although many participants believed that they were knowledgeable about sex, only a small number of them actually possessed accurate sexual knowledge. Few participants in this study, targeted to a specific age (eleventh grade), had used the Internet as a major source for obtaining sexual knowledge, in contrast to results from previous studies which covered a wider age demographic. Future research should include comparative studies between eleventh and twelfth graders as well as between those of the same age who are and who are not enrolled in high school. Additionally, it might be beneficial to consider the relationship between adolescent age and Internet use to study the influence of the Internet on sexual knowledge. Our results suggest that parents and schoolteachers should shoulder more responsibility in providing sexual education, particularly by improving their own knowledge and ability to provide such education, because many participants indicated that sexual knowledge should be acquired from parents and schools.