The current paper examined the impact of Dynamic Assessment (DA) on the speaking skills of EFL learners applying socio-cultural theory, which was developed by Vygotsky (1934). The study also tried to compare the results from online synchronous DA sessions, which were done in the form of one-to-one online training over Google Meet, with the results from asynchronous DA sessions, in which the trainer and the trainees were interacting over Gmail. To this end, 90 EFL institutional students from Iran, who were getting ready to take the IELTS test, were selected as the participants of the program. The cases were then divided into two experimental groups and one control group, each with 30 contributors. IELTS speaking tasks, marked by IELTS speaking band descriptors, were used to assess their speaking skills. Comparing the pre-test and post-test, the findings indicated development in all four areas of fluency, vocabulary, grammar, and pronunciation. Moreover, comparing the results from the two experimental groups showed that the participants who received asynchronous DA sessions outperformed the other experimental group. It is worth mentioning that a broader level of development was observed in experimental groups than the control group, in which the members received synchronous non-Dynamic Assessment (non-DA) methods.