This study was explored to seek the effect of mobile-mediated dynamic assessment on Iranian EFL learners' writing skill. The participants were 60 EFL learners from one English language institute in Shiraz, Iran, learning English at the intermediate level. Their age ranged from 15 to 30. The participants were randomly assigned to three equal groups (i.e., one control group and two experimental groups). The instruments and materials used in this study were oxford quick placement test (OQPT), the DIALANG writing test, WhatsApp, Google Docs, and the interview. To measure the participants’ general English proficiency level before the experiment, the participants took the Oxford Quick Placement Test (OQPT). The quantitative data were analyzed using using Kolmogorov-Smirnov and Shapiro-Wilk tests of normal, descriptive statistics, paired-sampled T-tests, ANOVA, and Tukey post hoc tests. For the qualitative data analysis, the answers to the interview questions were coded and analyzed using a typology of mediation. The ANOVA test showed that there were significant differences among the groups which implies that mobile-based dynamic assessment can enhance the writing proficiency among the language learners. In other words, both T and V groups in intermediate level had a significant progress in their writing proficiency. In light of the qualitative data, nearly all of the participants supported DA due to its advantageous benefits. They felt that this approach saved them time and increased their confidence and enthusiasm for writing, which is a challenging ability.