This qualitative study examines the professional agency of secondary English teachers in Midwestern South Korea. Specifically, it investigates how secondary English teachers in South Korea understand their professional agency and what mediates their professional agency. The ecological approach in this study recognises that agency encompasses both individual and environmental dimensions and is formed through the constant interplay between the individual and the environment. The dataset for this study comprises 15 semi-structured interviews with secondary English teachers in South Korea. The thematic analysis highlights a significant gap between Korean English teachers' espoused agency and realisable agency, as well as three levels of legitimation that teachers have to negotiate. These levels are individual, collegial and sociocultural. The findings from this study suggest that the participating teachers censored their own pedagogical behaviours due to their own sense of inadequacy and in response to structural requirements as well as the external evaluation of parents and high-stakes test scores. This study concludes with implications for English teachers in South Korea, theorisation on teacher agency and future research.