Teacher agency as an influential factor in teacher professionalism has recently gained global inquiry in the EFL context. However, no valid instrument has ever been designed to evaluate EFL teachers' ecological agency. This gap prompted the researchers of the current study to develop and validate a questionnaire to assess EFL teachers' ecological agency. In the first phase, a comprehensive review of the literature and semi-structured interviews were conducted to determine the underlying components of the teacher ecological (TEA) questionnaire based on Priestley et al.'s ("Flip the System" 134-148, 2015) ecological agency model. In the second phase, the newly developed TEA questionnaire including 40 items was subjected to reliability and validity issues. Therefore, it was piloted with 222 Iranian EFL teachers selected through non-probability convenience sampling. The Cronbach alpha results confirmed an acceptable reliability index (0.858). The results of factor analysis revealed that the number of items was reduced to 37 and indicated that the data on teacher ecological agency loaded on 3 components: iterational (9 items), practical-evaluative (14 items), and projective (10 items). In addition, the structural equation modeling (SEM) results confirmed that the model enjoyed sound psychometric properties. The upshots of the current study have undoubtedly significant implications for teacher educators and teaching practitioners.