Teacher assessment identity (TAI) is an integral element of quality assessment practice and professionalism, which has been widely overlooked in teacher education research. To bridge this gap, the present study explored the dynamism of TAI through an explicit training course. To do so, 22 novice and experienced Iranian EFL teachers completed a questionnaire on TAI before and after the course. Then both novice and experienced teachers attended a 16-hour treatment in which various assessment dimensions were taught. The participants also completed tasks reflecting TAI components. Moreover, five novice and five experienced EFL teachers were interviewed. The results of the independent samples t-test indicated that novice and experienced Iranian EFL teachers did not significantly differ regarding their assessment identity before and after the treatment despite improvements in their mean scores. However, the results of paired samples t-test revealed that the formal intervention has developed the TAI of both novice and experienced groups. This was substantiated by qualitative findings which demonstrated that, owing to the training, novice teachers manifested TAI in 16 domains while experienced teachers showed their identity as assessors in five domains. The study offers promising implications for EFL teachers and trainers, who can realize the dynamism of TAI and its core dimensions in light of instruction.