Teacher leadership has attracted scholarly interest over the past decade because of its potential to build internal capacity for sustainable school improvement. However, the existing studies have mostly been descriptive and limited to qualitative studies, lacking in terms of showing whether and to what extent principal leadership might influence teacher leadership. The purpose of this study was to examine the effects of principals' learning-centred leadership in fostering teacher leadership and the mediating role of teacher agency in Turkey. The study conducted a cross-sectional survey design. Structural equation modeling was used for estimating relationships among variables in the conceptual model. Findings demonstrated that principals' learning-centred leadership had a moderate but statistically significant direct effect on teacher leadership and a significant and moderate effect on teacher agency. Findings also highlighted teacher agency as a significant mediator in the relationship between principals' learning-centred leadership and teacher leadership. The research confirmed that principals' leadership practices with specific emphasis on teaching and learning were important for enhancing a school culture in which teachers' participation in decisions and their enthusiasm for undertaking leadership practices are supported. Copyright © 2020 John Wiley & Sons Ltd.