Theoretically framed within Vygotskyan sociocultural theory of mind (SCT), Dynamic assessment (DA) is a new approach to classroom assessment offering mediation to help learners perform beyond their level of independent functioning. As a web-based qualitative inquiry into the nature of mediation in DA and its impacts on the participants’ private speech patterns, this study explored the role of private speech as a social and psychological tool to mediate learners' thinking and performance in online DA. The subjects of the study were two Iranian university students along with two English native speakers orally narrating a series of picture stories and having weekly DA mediation with the researcher via Skype. In the present study, the students' private speech markers’ typology emerged out of thematic analysis of oral narratives transcribed before and after DA mediation and it was presented as a criterion to evaluate the learners' progression towards self-regulation. The frequency of occurrence of the learners’ private speech markers was reported and interpreted based on the emergent typology to evaluate the possible changes in their use as the result of DA mediation. Meanwhile, the differences between native speaker participants and the learners’ use of private speech markers in oral narratives were highlighted and discussed. The results of the study indicated that mediation was effective in reducing the number of private speech markers in the learners' narratives after mediation which reflects their progression towards self-regulation in online DA. The findings of the study highlight the importance of private speech as a mediating tool in DA which reflects learners’ struggle to take control of direction of learning and move beyond their current capabilities.