The numerous and varied applications of mathematics to all human endeavours justifies placing emphasis on the teaching and learning of the subject. This study established the effectiveness of the 5E instructional model (Bybee, Taylor, Gardner, Scotter, Powell, Westbrook, & Landes, 2006) for enhancing learning outcomes in mathematics. The study adopted a pretest-posttest, quasi-experimental design method. Simple random sampling technique was adopted to select 172 participants (96 males, 76 females, M = 15 years) for the study. Four research instruments were used. Data were analyzed using descriptive and independent t-tests. There was no difference in students' achievement and interest in mathematics before treatment. There was a significant posttest effect of treatment on students' mathematics achievement t(170) = 4.45, p < 0.05 and interest t(170) = 4.22, p < 0.05. Teachers are encouraged to adopt constructivist instructional approaches that discourage rote memorization and guide learners to develop their own understanding.