Child sexual abuse has become a significant public health concern in Vietnam in recent years, and the likelihood of being abused could be prevented by increasing the awareness of sexual abuse and self-protection skills among children. However, little is known about the perception and attitude of schoolchildren toward this issue in Vietnam. This study aimed to evaluate the knowledge and attitude of school-age children toward child sexual abuse and the risk factors affecting their knowledge and attitude. A cross-sectional study was conducted among 800 Vietnamese students from grades four to nine. Most of the respondents had insufficient knowledge of sexual abuse in children; teachers and strangers would not be perpetrators (57.9% and 74%); and schools and home were safe places (55.8% and 58.8%). Almost all participants disagreed with touching and non-touching actions, even from acquaintances (94.5% to 99.5%). Being female, older age, not living with family or relatives, and living in an urban setting were found to be positively associated with the right perception and attitude toward child sexual abuse. A sexuality education program should be officially applied at schools for children with the support of their parents to narrow the knowledge gap between different geographical locations and genders.