This study examined the effect of the ZPD-based discourse scaffolding on EFL learners' co-construction of L2 metadiscourse performing collaborative writing tasks and explored the discourse scaffolding dynamics. The participants were 160 EFL students that were assigned to four different treatment conditions: (i) formal teaching, (ii) input enhancement, (iii) non-ZPD interaction, and (iv) ZPD-based L2 discourse scaffolding. The ZPD learners were required to audio-record their task-focused social interactions and also write four weekly compositions and one delayed essay. The results showed a substantial effect for the ZPD-based L2 discourse scaffolding on the ZPD participants' use of English metadiscourse. Further, the microgenetic analysis of the social interactions revealed that the ZPD groups employed manifold scaffolding dynamics at various ZPD levels to expedite task performance and their peers' use of L2 metadiscourse. The asymmetrical discourse construction grew mutual with the less capable peers offering increasingly more self-initiated repairing moves within their activated ZPD. Also, the students' essays were rated as more reader-friendly over time. Finally, the implications for L2 research and pedagogy are discussed.