The purpose of this study was to determine the contribution of academic motivation and gender to academic performance among secondary school students attending day schools within Nakuru municipality. The study was based on the Self-Determination Theory by Ryan and Deci which explains two main components of human motivation; (a) humans are motivated to maintain an optimal level of stimulation, and (b) humans have basic needs for competence and personal causation or self-determination. The theory indicates that when these needs are met for the students, they will be intrinsically motivated and will actively engage in learning. The study utilised a mixed approach and adapted the ex-post facto design. The target population comprised all Form Two and Form Four students in the seventeen secondary schools in Nakuru municipality from which seven day schools were sampled using the stratified random sampling technique. The sample was made up of 489 students. Data was collected using a students’ questionnaire. The academic performance scores were obtained from the school records for two terms. Cronbach's alpha was used to estimate reliability and expert judgment technique was used to determine validity. The major statistical methods used in this study were: Pearson’s r, analysis of variance (ANOVA), and Tukey’s HSD post hoc test. The level of significance used for statistical tests was .05. The data was analyzed using Statistical Package of Social Science (SPSS). Descriptive statistics, means, frequencies, percentages and standard deviations were used for data presentation and to explain the variables in this study. The findings of this study indicated that there was a positive relationship between academic motivation and academic performance. Gender was found to strongly contribute to the learners’ academic performance. These findings will assist teachers, school administrators and other stake holders to engage in interventions in school and at home, that can improve the quality of learning and hence boost the students’ academic performance and to boost academic motivation.