Converging evidence indicates that argumentative thinking and writing are promoted by peer discourse. Here it is proposed that collaborative writing can serve as a bridge from peer discourse to individual argumentative writing. Three groups of sixth graders participated in a study that tested this hypothesis. Two of the groups took part in a month-long intervention that emphasized engagement and practice in dialogic argumentation. These two groups differed only in addition of a joint writing activity in one group. The third group served as a non-intervention control. Final individual essays showed superior performance by the joint writing group. Essays of the superior group more frequently included evidence-based arguments. They also more often integrated belief-incongruent statements with belief-congruent ones. Group differences transferred in several respects to essays on a new topic. Results support the claim that engagement in collaborative writing offers a promising path in developing argumentative writing.