This study investigated the effects of computer simulation and blended learning instructional strategies on senior secondary school students’ academic achievement in Geometry concepts in Abak Education Zone, Akwa Ibom State, Nigeria. The study adopted a non-randomised pretest–posttest control group quasi-experimental design. The population comprised 3,820 Senior Secondary School Three (SS3) Mathematics students, while a sample of 171 students (79 males and 92 females) was drawn from three co-educational public secondary schools using purposive sampling. Three research questions were formulated and three null hypotheses were tested at the 0.05 level of significance. Data were collected using a researcher-developed Geometry Achievement Test (GAT), which was validated by experts and found to be reliable at a Kuder–Richardson Formula 20 (KR-20) coefficient of 0.88. Students were assigned to two experimental groups and one control group and were taught using validated lesson packages in their intact classrooms. Data were analysed using descriptive statistics (mean and standard deviation) and Analysis of Covariance (ANCOVA), with pretest scores as the covariate. The findings revealed that students taught Geometry using the blended learning strategy significantly outperformed those taught using computer simulation and conventional teaching strategies. Students in the computer simulation group also performed significantly better than those in the conventional group. With regard to gender, no significant difference was found between male and female students taught using the computer simulation strategy. However, female students achieved significantly higher than male students in the blended learning group. It is recommended that Mathematics teachers integrate blended learning and computer simulation strategies into Geometry instruction to enhance students’ conceptual understanding, engagement, and academic achievement.