The aim of the study was to establish the effect of teachers' lesson observation practices on learners' academic achievement in Public Secondary Schools in Nandi East Sub-County, Kenya. The study was anchored on Edwin Locke's Theory of goal setting. The study adopted both descriptive survey research design and correlation research design. The study targeted 336 respondents, which comprised 30 principals, 30 deputy principals, 274 teachers, one Quality Assurance and Standards Officer, and one Director of Education in the sub-county. The sample size was 152 respondents, while the study adopted simple random and purposive sampling techniques. The study used both primary and secondary data. Data were collected using a questionnaire, interview-guide, and documentary analysis guide. The study adopted content validity, which was tested using experts' opinions. The study used Cronbach's Alpha method to test reliability. Data collected were analyzed using percentages, frequency distribution and mean. Inferential statistics were used to determine the relationship between the dependent and the independent variables using Chi-square. Results were as follows: lesson observation practices of teachers had χ2 = (283.718, p<0.05). The study concluded that teachers' lesson observation practices contribute to improvement in learners' academic achievement. The study recommended that lesson observation practices should be done regularly as it helps to improve teachers' school attendance.