This study investigated principals’adminis -trative effectiveness and students’ academic performance in public senior secondary schools in South West, Nigeria. Four research questions and four null hypotheses guided the study. The study adopted correlational survey and ex-post facto research designs. From a target population of 134 public senior secondary schools and 7,412 teachers, a sample size of 67 public senior secondary schools and 364 teachers was drawn from Ondo, Osun, and Ogun States using a multistage sampling procedure.Aresearcher-developed instrument titled Principals’ Administrative Effectiveness Questionnaire (PAEQ) and a Students’ Academic Performance Proforma (SAPP) were used for data collection. The reliability coefficient of 0.83 was obtained for the PAEQ using the split-half method, Pearson Product Moment Correlation, and Spearman Brown Prophecy Formula. Mean scores and standard deviations were used to answer research questions, while linear and multiple regression analyses were employed to test the null hypotheses at a 0.05 alpha level. The findings revealed that principals were effective in the administration of curriculum and instruction, school plant and facilities, and student personnel; and that students’ academic performance showed a good trend in SSCE results from 2013 to 2022. The study concluded that principals’ administrative effectiveness significantly influences students’ academic performance in public senior secondary schools in South West,Nigeria. It was therefore recommended that school principals should intensify administrative efforts towards structuring students’ learning activities to align with academic goals and expected outcomes in external examinations. Furthermore, principals should collaborate with State Post Primary Education Boards in Ondo, Osun, and Ogun States to ensure the adequate provision and maintenance of school plants and facilities to enhance the teaching– learning process and improve overall academic performance in public senior secondary schools