Class size is an essential factor in student academic achievement. Small class sizes improve student achievement and teacher morale. For this reason, the researchers of this study set out to determine the relationship between class size and academic achievement among students with learning difficulties in secondary schools in Kakamega County, Kenya. The researchers used descriptive survey and correlational research designs. The target population was 36,453 third form and 37,532 fourth form students (eleventh and twelfth grade); 1,288 class teachers; and 12 sub-county education directors. Stratified random, purposive, and saturated sampling techniques were used. Data collection tools included questionnaires, interview schedules, and focus group discussion guides. Data were analyzed using percentages, means, standard deviations, Pearson's (r) correlation and ANOVA. Findings indicated a statistically significant relationship between class size and academic achievement among underachieving students. Findings show that increase in class size leads to decrease in academic achievement among these students.