Introduction: One in 10 children would be sexually abused before their 18th birthday; about one in seven girls and one in 25 boys would be sexually abused before they turn 18. The statistics continues to increase even as low and middle-income nations of the world are gradually coming out to report cases of child sexual abuse. Aim: This study aimed to assess the knowledge and perceived effect of sexual abuse among adolescents attending selected secondary schools in Mushin Local Government. Methods: 414 respondents from selected secondary schools in Mushin Local Government participated in the study using the multistage random sampling technique. A self-designed questionnaire was used to collect data. Data were analyzed using Statistical Package for the Social Sciences version 26 and bivariate analysis was conducted using Chi-square test. Results: More than half of the adolescents fall within the age range of 15–17 years and majority were females. In addition to this, just a little above half of the participants were Christians and just below half of the students were in SSS 2. Also, more than half of the participants were Yorubas and well above half were from a nuclear family. Just about half of the adolescents have good knowledge about sexual abuse. In the measures of association, the relationship between the level of knowledge of sexual abuse and the perceived effect of sexual abuse was statistically significant. The relationship between adolescents’ perception of the effect of sexual abuse and their concept of the common forms of sexual abuse was also statistically significant. Conclusion: This study revealed that the adolescents under study had knowledge about sexual abuse and this knowledge determined their perception of the effect of sexual abuse, and this in turn determines what they consider as child sexual abuse. It is therefore necessary to intensify efforts on educating and re-educating children and adolescents on sexual abuse, how to identify it and how to prevent it through campaigns and health promotional activities.