Academic achievement is important in the determination of a students' future prospects. Educationists and other stakeholders have consistently used students' academic achievement results as basis of advancement and employment opportunities. The learners' academic self-esteem correspondingly shape the learning environment of students. The purpose of this study was to examine students' academic self-esteem and academic achievement relationships in public secondary schools in Nakuru County, Kenya. Correlational research design and both quantitative and qualitative research approaches were used in the study. The population of the study comprised 23,309 from 29 public secondary schools. The sample size for the study was 407 respondents, comprising of 378 form three students and 29 class teachers. The study used closed-ended questionnaires to collect data on selfesteem from students and interview schedule to collect data from class teachers. Self-esteem scale was adopted and adjusted using Rosenberg Self-esteem Inventory. Data for academic achievement was obtained from school examinations results record. Cronbach's Alpha test of internal constituency for self-esteem questionnaire had reliability coefficients of 0.873 which was considered acceptable. The study found out that there was a significant relationship between students' academic self-esteem and academic achievement in public secondary schools in Nakuru County, Kenya. The study concluded that students' academic self-esteem affects their academic achievement. In respect to this, the study recommended the Kenya Institute of Curriculum Development (KICD) to develop a curriculum or teaching methods that seeks to improve students' academic self-esteem for better academic achievement.