This study investigated whether there were any changes in knowledge and normative beliefs regarding abstinence and condoms and personal risk perception of acquiring HIV among Zambian secondary-school students who were exposed to a peer sexual health intervention. Schools were randomly assigned to intervention and control groups. A baseline assessment that measured knowledge, normative beliefs about abstinence and condoms and personal risk perception was conducted in all schools. Students in the intervention group were exposed for a 1-hour-and-45-minute-long in-class peer sexual health intervention. Students in the control schools were exposed to a 1-hour-long in-class peer water purification intervention. A follow-up assessment of students was conducted after the intervention. Logistic regression analyses were conducted to determine whether significant differences between intervention and control groups emerged at follow-up. Relative to students in the control group, knowledge was higher and normative beliefs about abstinence and condoms were significantly more positive among students in the intervention schools. Students in the intervention schools also had a higher personal risk perception of acquiring HIV than students in control schools. These findings show that a school-based peer sexual health intervention implemented in Zambia was effective in increasing knowledge, positive normative beliefs about abstinence and condoms, and personal risk perception.