The study examined the predictive validity of junior secondary school certificate Mathematics and Integrated Science examinations on academic performance of senior secondary school Mathematics students in Ondo State. Two research hypotheses were generated to guide the study. This study employed expost-facto design. Thirty six secondary schools were purposively selected from six Local government areas of the three senatorial districts (Ondo North, Ondo Central and Ondo South). Two Local Government Areas were selected from each of the senatorial districts using simple random sampling techniques. A total of 1,826 2020/2021 SSS3 Mathematics students that came in by JSSCE, had complete record from SSS1 to SSS3, sat for Mathematics in SSS classes and also sat for 2020/2021 WAEC/NECO SSCE were involved in the selected secondary schools. Data collected were analysed using correlation analysis, regression analysis as well as analysis of variance. All the results revealed that JSSC Mathematics examinations and JSSC Integrated Science examinations significantly predict academic performance of students at the SSS2 unified examination in Mathematics, WAEC SSC examinations in Mathematics and NECO SSC examinations in Mathematics (p < 0.05). Based on the findings of this study, it was therefore recommended among others, that JSSCE results should remain a yardstick for admitting Mathematics students into senior secondary schools. Keywords: Academic, Predictive, Performance, Validity, Correlation,