This study investigated the effects of teacher-student ratio on academic performance of selected Secondary school students. The study was conducted at Changamwe constituency, Mombasa. The targeted population was public secondary schools in Changamwe constituency with 3800 respondents. This research was led by descriptive survey research design, which employed a simple random sampling method in the selected 3 senior secondary schools in Changamwe. Four research questions and hypotheses were formulated to guide the study. The researcher used questionnaires as the major instruments in collecting the data. 3 principals 15Teachers, and 114 students from a sample size of 132 respondents. The following objectives served as the basis for the research. To examine the teacher student ratio and how it affects the academic performance of the students, to investigate how variations in teacher student ratio correlate with academic performance among students in the division, to examine teachers workload in relation to the student population exploring how it may affect teaching quality and student outcomes and finally to investigate the level of student engagement and participation in classes, considering the impact of teacher student ratio on these factor Pretesting was place in one school and the study tool was a questionnaire. The findings showed that teacher student ratio affects academic performance of the students were schools with lower teacher student ratio consistently showed higher level of student achievements. Schools with higher teacher student ratio found reported low academic performance of the students. The findings revealed that teachers who handled an excessive workload of high student ratio and about 6 classes and above tend to produce low score. they reported challenges in lesson planning and assessment. The study also reported that student engagement level was higher in classrooms with smaller teacher-student ratio. The study concluded that the teacher student ratio has a significant effect on students’ academic performance in public secondary schools in Changamwe constituency. Schools with lower teacher-student ratio reported better performance due to improved interaction, high engagement, low teachers’ workload. Based on the research findings, the following recommendations are proposed to improve the academic performance of secondary school students; reduce teacher student ratio. policymakers should consider increasing staffing in schools to achieve more manageable classes, school administrators should implement policies that distribute teaching assignments more evenly, Enhancing student engagement and participation in class by adopting strategies such as interactive learning technologies, peer discussion and extracurricular academic activities to improve student engagement. Finally, schools should invest in teacher training so that to improve teaching quality of the teachers. this can be done through professional development.