jats:p This article investigates the effect of concept-based instruction (CBI), a pedagogy based on Vygotsky’s sociocultural theory, on second language (L2) learners’ writing development. A quasi-experimental intervention lasting 16 weeks was conducted involving 88 first-year Chinese university students of English. The participants were assigned to three conditions: Full CBI (F-CBI) treatment, Partial CBI (P-CBI) treatment (i.e. CBI without peer–peer talk) and control. A definition task and two writing tasks (a narrative and an argumentative task) were used to measure students’ knowledge and understanding of genre and their writing performance, respectively. The results indicated that the F-CBI intervention generated significantly larger gains in learners’ understanding of genre as well as their overall writing performance than did the control group. In addition, a significant difference was also observed between the F-CBI group and the P-CBI group in the definition task and the argumentative task, which further suggests that peer–peer talk plays a crucial role in enhancing learners’ conceptual development. This study highlights the feasibility and effectiveness of integrating CBI in L2 writing instruction and the importance of engaging learners to talk through their understanding and use of concepts in the classroom. Theoretical contributions to Vygotsky’s sociocultural theory of mind are also discussed. </jats:p>