Purpose: Although action research (AR) is considered essential for teacher development in second/foreign language (L2) education, no validated scale has been designed for its measurement. With that in mind, this study attempted to develop and validate an Inventory on Teachers' Beliefs about Action Research (ITBAR). Design/methodology/approach: For this purpose, a model construction and validation framework consisting of exploratory and confirmatory analyses was used to examine the construct validity of a proposed model. The participants were 287 practicing English language teachers at different institutes, schools, and colleges/universities in Iran. Findings: The results of the exploratory factor analysis indicated that the scale can be best explained by six factors. However, one factor was removed from further analysis because of the low reliability of its components. Thus, confirmatory factor analysis was run with the five remaining factors which, based on the loading of the items and their underlying themes, were named Teacher Empowerment, Practical Issues, Professional Development, Institutional Culture, and Research Engagement. Originality/value: The ITBAR developed in this study is aimed to reflect the realities of L2 classrooms and give direction to the process of critical reflection by providing a context through which teachers articulate their own beliefs about AR.