From a writing as a social practice approach, we explore the construction of academic identities in undergraduate dissertation writing at a Chilean university. An online questionnaire was applied and interviews were conducted with students from four disciplinary areas. The participants perceive the dissertation process as an enriching learning experience, but also as lonely and unmotivating. We also observed three forms of identity negotiation that facilitated or hindered engagement with and motivation toward the dissertation: the negotiation of diverse trajectories, of the student’s own voice, and of the disciplinary field. From our findings, we make recommendations for the development of dissertation writing programs. Lastly, this study describes various forms of identity negotiation that play a key role in students’ engagement in their dissertation process.