In Thai society, teenagers having sex before marriage is often avoided as it is considered forbidden. Nonetheless, it is widely believed that young people are becoming more sexually active before marriage. This study aimed to determine factors associated with attitudes toward premarital sex among Thai adolescents. Secondary data was used and drawn from the 2011 Survey on Conditions of Society and Culture by the National Statistical Office Thailand. The total study population was 674 Thai adolescents, comprising 364 younger adolescents aged 13–16 and 310 older adolescents aged 17–19. Three-quarters of the sample were school-going students. The analysis used Pearson chi-square and binary logistic regression. The results found that age, gender, education status, and acceptance toward sexual behaviors had a significant association with attitude toward premarital sex with those under 15 years old (p < .05). The probability of agreement with premarital sex was highly associated with the acceptance of open sexual behavior. However, older age groups and school-going adolescents had less likelihood of agreeing with premarital sex when compared with younger and out-of-school groups. Female adolescents had less probability of agreeing with premarital sex than males. In conclusion, accepting open sexual behavior is at risk of agreeing with premarital sex, but education programs in school may protect them from it. To provide knowledge and prevent the risk of premature sexual activity, sexual health should be promoted, including the appropriate sexual value, in primary school programs. In addition, the government should encourage sexual health through social activities for out-of-school adolescents.