BackgroundAlgebra remains a challenging aspect of the secondary school mathematics curriculum in Nigeria, contributing to persistent learning difficulties and poor achievement.ObjectivesThis study examined the effect of Gagné's instructional strategy on students’ performance and retention in Algebra, as well as its impact across gender.MethodsA quasi-experimental design was employed involving 382 students selected through multi-stage sampling. An Algebra Performance Test with a reliability coefficient of.69 was used for data collection. Descriptive statistics and Quade's non-parametric ANCOVA were used to analyse the data.ResultsResults showed that students taught using Gagné's instructional strategy performed significantly better than those taught through the conventional approach, F(1, 380) = 518.11, p < .001, partial η2 = .577, indicating a large practical effect. Gagné's instructional strategy also supported retention outcomes, reflecting sustained learning over time. Additionally, no significant gender differences were found in performance after adjusting for pretest scores, F(1, 181) = 0.16, p = .690, partial η2 = .001, suggesting that Gagné's instructional strategy benefits male and female students equally.ConclusionThe findings demonstrate that Gagné's instructional strategy effectively enhances both immediate and sustained Algebra learning. It is recommended that Gagné's instructional strategy be integrated into mathematics instruction and teacher training programmes to improve learning outcomes in secondary schools. Implications of these findings and recommendations for practice are discussed.