The purpose of this study is three-fold. First, to explore the effects of online peer-dynamic assessment (Peer-DA), rooted in the Zone of Proximal Development (ZPD) notion, on boosting Iranian high school students' writing skills. Second, to disclose how online Peer-DA can lead to improving the participants' writing skills. Third, to disclose their perceptions of the effectiveness of online Peer-DA in boosting writing skills. To these aims, one intact grade 11 class, including female students (n = 18) was selected randomly. The class went through pre-test, intervention, immediate post-test, and delayed post-test procedures. Additionally, the peers' interactions were recorded meticulously and one-third of the students were selected randomly and invited to a focus-group interview. The data were analyzed through a repeated measure ANOVA, a microgenetic development approach, and a content analysis approach. Findings evidenced a significant improvement in the students' writing skills on the immediate post-test and the delayed post-test. Also, the microgenetic development analysis documented that the contingent prompts led to improving the students' writing performance. Besides, the findings of the focus group interview evidenced that the students had positive attitudes toward the online Peer-DA. Finally, a range of implications is offered for relevant stakeholders.