This study examined factors related to sexual assault among early adolescents in Japan. During a 6-month period, an anonymous questionnaire survey was distributed to 1,246 students ages 13-15 years at eight junior high schools. Consent to participation in this study was obtained from a total of 1,125 (90.3%) students. It was found that the prevalence of sexual assault was higher among both female and male students who had sent a message/photo to or met a person they initially contacted via the Internet, while the prevalence was lower among students with a conservative attitude toward sexual activity. In addition, female students who recognized the signs of dating violence had less experience of sexual assault. These findings indicate the need for increased education about the Internet and dating violence for effective prevention of sexual assault.