The study aims to examine the effectiveness of mobile-mediated Dynamic Assessment using a WhatsApp group to improve student's writing skills in English for Specific Purposes classrooms. Furthermore, it also investigates the students' responses toward the mediating practices through the Dynamic Assessment activities. Thirty-one students of the English Diploma program in a public university in Indonesia participated in the study. The study employed a mixed-method explanatory sequential design. The data were gathered through pre-test, post-test, and observation of online records. A paired sample t-test and an independent sample t-test were utilized to find out the significance. The Reciprocity Framework was used to analyze the students' responses to the mediating practices. The results show that mobile-mediated Dynamic Assessment effectively enhances students' writing skills. Furthermore, the reciprocity analysis showed that the mediating process provides a convenient and less intimidating platform to respond to the teachers' feedback. Moreover, the activity provides a platform for students to collaboratively improve their mastery in composing writing.