This study investigates the effect of Scaffolding Instructional Strategy (SIS) on seniorsecondaryschoolstudents achievement and retention in geometry in Katsina State, Nigeria. Mathematics remains a core subject in the Nigerian curriculum, yet students consistently record poor performance, particularly in geometry, as reported by WAEC and NECO. The persistent failure rates have been linked to ineffective teaching methods, negative student attitudes, and gender-related differences in learning outcomes. The study adopts a pre-test, posttest, post-posttest quasi-experimental design with two groups: an experimental group exposed to scaffolding and a control group taught with conventional lecture method. A sample of 120 SSII students drawn from two co-educational secondary schools in Katsina Education Zone participated. Data were collected using a researcher-developed Geometrical Concepts Performance Test (GCPT) validated by experts and analyzed using descriptive statistics and t-tests at 0.05 significance level. The assumed results indicated that scaffolding significantly improved students’ achievement and retention in geometry, with no significant gender differences observed. The findings provide insights for teachers, curriculum planners, and policymakers towards improving pedagogy, reducing failure rates, and enhancing students’ long-term mastery of geometric concepts.