This study was intended to explore the effect of an interactionist model of dynamic assessment (DA) on Iranian EFL adult learners’ intrinsic motivation. One hundred participants were selected based on the convenience sampling procedure.The participants were assigned to the experimental ( n =50) and control ( n =50) groups. An interactionalist model of DA in reading comprehension was implemented in the experimental group seven times during the term. Data were collected through the Academic Motivation scale and were analyzed by an independent-samples t -test. The results indicated a significant difference between the two groups in terms of their amount of intrinsic motivation.It is concluded that incorporation of DA as a supplement procedure to classroom activities has apositive effect on EFL learners’ intrinsic motivation.