This study investigates the impact of principals' instructional leadership practices on secondary school teachers' effectiveness in the North Central Geo-Political Zone of Nigeria. To answer the research question of the study, the Principal Instructional Management Rating Scale (PIMRS) and the Virgilio Teacher Behaviour Inventory (VTBI) were administered to 389 teachers in 18 public secondary schools. The results indicate that instructional leadership, in terms of defining school mission; managing instructional programs; and developing a positive school learning climate, are significantly and positively associated with teachers' effectiveness. Therefore, stakeholders in the education sector can make informed decisions and interventions to enhance teachers' effectiveness through the principals for desirable learning outcomes. Similarly, school principals should focus more on instructional leadership practices that relate to teaching and learning in order to improve teachers' effectiveness. This study adds to the existing knowledge on factors that determine teachers' effectiveness in Africa, in particular, in Nigeria.