Student poor performance in mathematics has been a serious concern to mathematics educators and all educational stakeholders in Nigeria. The instructional strategies used by teachers in teaching mathematics have been identified by researchers as one of the problems behind this poor performance in mathematics. If this poor performance of students in mathematics continues it could deny the realization of educational goals for scientific and technological development of the nation. This research therefore, was carried out to examine the effects of multimedia instructional strategy on senior school students' performance and retention in mathematics in Ilorin, Kwara State, Nigeria. The study was a quasi-experimental type of the pre-test, post-test, non- randomized, control group design. The design is a 2x2×1 factorial design. A sample of 81 senior secondary school II students from two secondary schools in Ilorin south local government area of Kwara State was involved. The research instruments were; developed multimedia instructional package. The experimental group was taught using multimedia instructional strategy, while the control group was exposed to the conventional teaching. Data collected for analysis were subjected to ANCOVA and t-Test statistics. From the study, findings showed that: (1) there was a significant difference between the experimental groups taught mathematics using multimedia and the control group taught mathematics using conventional method (2) both female and male students were capable of competing in classroom activities when taught with a better instructional strategy (3) multimedia instructional strategy improve and stimulate students' retention level thus, multimedia instructional package helps to concretize the learning of mathematics and makes learning more effective. Based on the findings of this study, it was recommended that multimedia instructional strategy may be integrated into mathematics curriculum and also used to arouse students' interest in mathematics.