With the growing concern for the issues and problems associated with static testing, dynamic assessment (DA) has been recently receiving increased attention as an alternative in educational settings. The present single case study attempted to investigate the potential of interactionist dynamic assessment in developing an Attention Deficit Hyperactivity Disorder (ADHD) L2 learner's reading competence and selective attention. A 13-year-old female EFL learner with the combined type of ADHD voluntarily participated in this study. During the DA sessions, the learner read some short passages and answered the questions through mediations emerging from the interactions between the mediator and the learner. The qualitative analysis of DA protocols led to the development of an inventory of different forms of implicit and explicit mediations. Moreover, the quantitative analysis of the data showed that DA contributed to the learner's reading comprehension and some congruent parameters of selective attention. The results of the present study call for teachers', policymakers', and material developers' increasing attention to the mediations required to assist ADHD L2 learner's independence from the mediator, which demonstrates self-regulation.