In this research, we developed and empirically tested a dialogic writing intervention, an integrated language approach in which grade 5/6 students learn how to write, talk about their writing with peers, and rewrite. The effectiveness of this intervention was experimentally tested in ten classes from eight schools, using a pretest-posttest control group design. Classes were randomly assigned to the intervention group (5 classes; 95 students) or control group (5 classes; 115 students). Both groups followed the same eight lessons in which students wrote four argumentative texts about sustainability. For each text, students wrote a draft version, which they discussed in groups of three students. Based on these peer conversations, students revised their text. The intervention group received additional support to foster dialogic peer conversations, including a conversation chart for students and a practice-based professional development program for teachers. Improvements in writing were measured by an argumentative writing task (same genre, but different topic; near transfer) and an instructional writing task (different genre and topic; far transfer). Text quality was holistically assessed using a benchmark rating procedure. Results show that our dialogic writing intervention with support for dialogic talk significantly improved students' argumentative writing skills (ES = 1.09), but that the effects were not automatically transferable to another genre. Based on these results we conclude that a dialogic writing intervention is a promising approach to teach students how to talk about their texts and to write texts that are more persuasive to readers.