This study investigated the effects of Flipped Classroom Strategy (FCS) on students’ academic achievement in Mathematics in Senior Secondary School 11 students in Obio/Akpor Local Government Area, Rivers State. A sample size of one hundred and four (104) students; fifty three (53) students (34 boys and 19 girls) in the control group and fifty one (51) students (34 boys and 17 girls) in the experimental group was usedfor this study. Purposive sampling technique was used to compose the sample. A pre-test post-test quasi experimental design was used. One instrument; Mathematics Achievement Test (MAT) was developed by the authors and was validated by mathematics teachers and experts in the field of measurement and evaluation. To test the internal consistency of the MAT, the Kuder Richardson technique was used and a reliability analysis produced a Kuder Richardson of 0.701. The obtained data was analysed for means and standard deviations, while t-test and analysis of covariance (ANCOVA) were used to test the hypotheses. It was found that FCS significantly improved students’ academic achievement in mathematics. However, the students’ academic achievement in mathematics for the two groups (FCS & TCM) was not improved significantly but was better for those taught using the TCM.