Drawing on an ecological approach to teacher agency, this empirical study investigated the dynamic interplay between teachers and the environment which emerges as teachers enact their professional agency in curriculum design. The dataset comprises semi-structured interviews with 15 Korean secondary English teachers. The findings from the thematic analysis indicate that teacher agency is achieved through the dynamic interplay between teachers and different ecosystems: micro-, meso-, exo-, macro- and chronosystem. This study concludes with practical and theoretical implications as well as suggestions for future research.