Believing that rape is acceptable in some situations may account for adolescent boys’ perpetration of forced sex on girls. This study was intended to examine two hypothesized cognitive factors of adolescents’ rape-supportive beliefs: general knowledge, measured with grade point average (GPA); and specific knowledge about sexuality, measured with a newly devised questionnaire. Fourteen-year-old adolescents (N = 248) participated in a short-term longitudinal study. They completed questionnaires designed to assess sexual knowledge and rape-supportive beliefs, and six months later completed them again. Sexual knowledge increased sharply between Time 1 and Time 2, whereas rape-supportive beliefs decreased during the same time. Boys obtained higher rape-supportive belief scores than girls. Regression analyses showed that sexual knowledge significantly predicted the level of rape-supportive beliefs six months later, independent of GPA and sex of participants. GPA accounted for a greater part of the variance in rape-supportive beliefs. This article discusses the importance of paying attention to the level of academic achievement of adolescents, as well as to their sexuality-specific knowledge, as a way of improving the efficiency of programs specializing in the prevention of adolescent sexual violence.