While previous studies on assessment as learning (AaL) in second language (L2) writing have mainly focused on writing teachers’ practices and perceptions of AaL, scant research has examined the relation between students’ engagement and writing performance in an AaL context. To fill the void, this study examined how students’ engagement related to their writing performance. Drawing on writing drafts, interviews, verbal reports, observation field notes, and documents, cross-case analyses of two focal students demonstrated that learner engagement in an AaL context was positively associated with improvements in writing performance. The student who demonstrated greater reciprocity in collaborating with teachers and peers in the AaL context, as well as proactivity in taking charge of her learning in L2 writing, showed greater improvements in content, organization, and language of argumentative writing.