Despite their experience, teachers continuously need to develop themselves professionally as the recency of the knowledge they acquire in teacher education programmes wears off in time -- which means they need to be autonomous enough take responsibility for their own professional learning. This study, therefore, defines 'teachers' professional autonomy' as teachers' capability, motives, and time to develop themselves professionally in collaboration with their colleagues; and it proposes Teachers' Professional Autonomy Questionnaire (TEPAQ) to measure the construct. First, two focus group interviews were conducted with 15 novice and experienced teachers, and an item pool was written. This initial version of TEPAQ was subjected to expert feedback sessions and piloting. The main data were gathered from 375 EFL (English as a foreign language) teachers working at state and foundation universities in 36 cities and 7 regions of Turkey. Following exploratory and confirmatory factor analyses and reliability analyses, TEPAQ was finalised with 5 factors and 23 items. TEPAQ can be used individually by teachers or institutionally by teacher educators or administrators with an aim to evaluate the strengths and weaknesses regarding teachers' or teacher trainees' professional autonomy and the areas they need support and assistance to take responsibility for their professional learning.