Background: Adolescents throughout the world are at risk of developing PTSD. PTSD has detrimental long term effects on every facet of life and has been shown to negatively impact the various areas of life. These areas often include: academic performance, cognitive abilities, emotion regulation and behavioural regulation. In adolescents, PTSD has particularly been implicated in high dropout rates which subsequently leads to poverty, fewer opportunities and a decreased lifespan. Objective: The purpose of this study was to establish the prevalence of PTSD among adolescent boys in selected secondary schools in Machakos County, Kenya. Methods: Two secondary schools were conveniently sampled with a total of 976. The adolescent boys were aged 14-17-years. The respondents filled a Socio-Demographic Questionnaire (SDQ) and the Child PTSD Symptom Scale-V-Self Report (CPSS-V-SR) which screened for PTSD symptoms. Results: The findings showed that the prevalence of PTSD was at 27.6%. The findings also showed that respondents who lived in other counties (exclusive of Nairobi County and Machakos County) had the least frequency of 42 (15.6%) but reported the highest mean PTSD symptoms at 24.5 (±24.5) and a statistically significant mean difference established in the counties p<0.006. Conclusion: The need for psychotherapy in secondary schools is therefore essential in order to alleviate PTSD prevalence. This is also crucial in order to buffer adolescents from comorbid diseases and psychological disorders.