This paper examined class size and academic performance of students in public and private secondary schools in Port Harcourt and Obio-Akpor Local Government Area of Rivers State. A correlational research design was adopted for the study. The population of this study was made up of all the 2,042 academic staff and 12,727 SS2 students of 37 public senior secondary schools in Port Harcourt and Obio-Akpor LGA of Rivers State, totaling 14,769. A sample size of 389 respondents were drawn from the entire population using Taro Yamane. The sampling technique was a proportionate stratified random sampling. Questionnaire and Teacher Made Test were used as the instrument to generate data. The test was designed to measure students’ academic performance. The research instruments were titled: Class Size Scale (CSS) and Teacher Made Test Scale (TMTS). The instruments items were designed and validated by the researcher. Kuder Richardson reliability method was used to test the reliability of the instruments. The reliability coefficients of Class Size Scale and Teacher Made Test Scale are 0.85 and 0.83 respectively. The findings revealed that that there is weak and negative significant relationship between large class and academic performance of students in public secondary schools in Port Harcourt and Obio-Akpor Local Government Area of Rivers State, this is linked to several challenges associated with it. Also, there is strong and positive significant relationship between small class size and academic performance of students in private secondary schools in Port Harcourt and Obio-Akpor Local Government Area of Rivers State. Based on the findings, it was recommended among others that there should be government mandate on large class size reduction in public schools for effective teaching and learning in order for students to achieve good performance in their studies. Consequently, more classroom should be built, teachers should be recruited to replace retired and resigned teachers to meet National Policy on Education (NPE) stipulation on student-teacher ratio.