Teacher agency (TA) has been increasingly supported as an influential factor for teacher professional learning, school improvement and sustainable educational change. Previous studies, however, feature a variety of discrepancies in their conceptualisation and approaches to examining teacher agency. A systematic review is essential to map the field, refine our understanding of the concept and inform relevant stakeholders of useful insights for policy making and further research. After being screened against a number of inclusion criteria, 104 empirical studies were selected to be included in this review. Thematic analysis was adopted for the data analysis of these papers. Findings indicate an imbalance in geographical, theoretical, methodological and thematic distributions in previous studies. Six major themes were identified from the literature: (1) TA enactment, (2) the role of teacher cognition, (3) influential factors, (4) implementing professional development interventions, (5) TA outcomes and (6) TA change trajectory. TA can be extrapolated in terms of its source, enactment and outcomes. Teachers' capacities, implementation strategies, and contextual factors jointly determine the achievement of their professional agency. A conceptual framework for accounting teacher agency holistically is proposed and implications for relevant stakeholders are discussed.