The purpose of this study was to investigate the effects of external instructional supervision on students' academic performance in KCSE in public secondary schools in Rongo Sub-County, Kenya. Specifically, the study aimed at achieving one objective viz: Determine the influence teacher classroom observation on students' academic performance in KCSE in public secondary schools in Rongo Sub-County. A descriptive survey design was adopted for this study. The target population comprised of 309 teachers from 16 public secondary schools in Rongo Sub-County. The sample size therefore consisted of 16 principals, 96 teachers and 2 quality assurance and standards officers hence constituting a total of 114 respondents that were used in the study. Data were collected using questionnaires and interview schedules.The validity of the instruments was done using a pilot study. The reliability of the instruments was computed using a formula by Mugenda & Mugenda (2006) that recorded a correlation coefficient of 0.789; thus, the instruments were reliable. Data analysis was run using percentages and frequency counts. The findings indicated that teacher classroom observation influenced students’ academic performance in KCSE in public secondary schools in Rongo Sub-County. The results further revealed a positive correlation between comments given to teachers by the supervisors and students’ academic performance in KCSE examinations and finally, that the selection and use of teaching and learning resources improved teacher delivery of content, which eventually translates into student academic performance at KCSE examinations. The study recommends that there should be enough provision of facilities to enhance the process of classroom observation by external supervisors