This study investigates the role of school principals' instructional leadership as a predictor of teachers' professional development. The research sample comprised 304 secondary school teachers and 19 principals in Awka South, Anambra State, Nigeria. The study adopts Hallinger and Murphy's (1985) Instructional Leadership Model. Data were collected using the "Principal's Instructional Leadership Practices and Teachers' Professional Development" (PILPTPD) tool. Correlation analysis was utilized to explore the relationship between school principals' instructional leadership and teachers' professional development, while multiple regression analysis was employed to ascertain the predictive capacity of instructional leadership on teacher professional development. The results reveal significant associations between instructional leadership and teacher professional development and growth. Ultimately, instructional leadership emerges as a significant predictor of teacher professional development. This study contributes to the expanding literature emphasizing the nexus between principal instructional leadership and teacher professional development, shedding light on the mediating role of principals' leadership in influencing teachers' professional development and instructional practices.