This study examines how supervisor-candidate coauthoring collaborations contribute to doctoral students' writer identity. Three candidates' coauthorship experiences with their supervisors were investigated in depth using a multiple-case study design. Interviews, written reflections, and email correspondence between coauthors enabled thick descriptions of these candidates' writer identity formation. Guided by Burgess and Ivanic's framework of writer identity, the multiple-case study showed how the candidates' autobiographical selves, discoursal selves, authorial selves, and perceived writer were influenced through the experience of coauthoring with supervisors. Notably, the candidates benefited from supervisor-candidate coauthorship by engaging in scholarly collaborations, bolstering their confidence as academic writers, and strengthening their authorial voice and rhetorical awareness. This study also reveals potential pitfalls or challenges of such collaborations, highlighting key considerations for supervisors and candidates considering coauthorship.