Background: Adolescent’s sexual abuse is any interaction between adolescents and an adult (or another adolescent) in which the adolescents is used for the sexual stimulation of the perpetrator or an observer. Sexual abuse can include both touching and non-touching behaviors. Abusers often do not use physical force, but may use play, deception, threats, or other forms of coercion to engage adolescents and maintain their silence. Objective: The objective of this study was to assess prevalence and associated factors of adolescents sexual abuse among adolescents in high school, Nekemte town, East Wollega Zone, Oromia Regional State, Ethiopia, 2017. Methods: School based cross sectional study was conducted in high school of Nekemte town, using systematic random sampling technique from March 1st to 15th, 2017. Data was collected from 377 adolescent students using structured, pre tested and self-administered questionnaire. After data collection, data was cleared, coded, checked and entered in to computer software SPSS version 20.0. Univariate, analysis was performed for frequency and percentage. Binary logistic regression analysis was done to identify factors associated with sexual abuse and these factors were taken in to multivariate analysis with 95% confidence interval. Result: Among study participants involved in the study 126 (33.4%) were sexually abused at least once in their life time and the remaining 251 (66.6%) were not sexually abused. Factors of sexual abuse associated significantly were grade attending, Fathers occupation and Mothers’ education and occupation positively associated with sexual abuse of adolescents while drinking alcohol and had sexual intercourse associated inversely. Conclusion and recommendation: The prevalence of sexual abuse is high and needs every one’s attention on its reduction. Awareness creation for families and communities on sexual abuse and education for adolescents on short and long impact of sexual abuse is recommended.