Education is an important ingredient in the progress and changes of countries the world over. The purpose of the study was to determine the influence of capacity building for teachers on students’ academic performance in public secondary schools in Kenya. The article is an extract from a study done in public secondary schools in the Yatta sub-county, Machakos County, Kenya. The study was guided by the instructional leadership model. This study adopted a descriptive research design, survey method. The study was conducted among 247 respondents comprising 19 principals and 228 teachers. The study employed stratified sampling techniques and simple random sampling techniques. Data collection was done using questionnaires for principals and for teachers. Data were analysed using Statistical Package for Social Sciences (SPSS). The findings revealed a strong positive correlation between the principal’s involvement in capacity building for teachers and students’ academic performance from the principal’s perspective. However, the results of the teacher’s questionnaires indicated a weak positive correlation between the principal’s involvement in capacity building for teachers and students’ academic performance. The study concludes that principals' capacity building for teachers positively influences the student’s academic performance. However, capacity building in most schools has not been implemented appropriately. It was; thus, suggested that all principals working in public secondary schools enhance capacity building for teachers to ensure that they are always motivated which will in turn help to improve the student’s academic performance. Capacity building should be done in consultation with the teachers for it to be effective. It should be a policy from the ministry of education that capacity building for teachers is improved in all public schools.