Abstract Medical students in Sweden often write their first thesis in English as a foreign language without prior training in academic writing. This study examines their writing beliefs and self-efficacy, and how these may change during the writing process. Writing self-efficacy refers to a person’s belief in their ability to perform specific writing skills and tasks, such as writing a thesis. Forty-two Swedish medical students (42/300, 14%) completed validated questionnaires examining their writing self-efficacy and beliefs regarding the writing process: The Situated Academic Writing Self-Efficacy Scale (SAWSES) and The Writing Process Questionnaire. Despite the medical students’ limited prior writing experience, they had a high writing self-efficacy, suggesting overconfidence. There was a slight tendency for the self-efficacy to decrease during the actual writing of the thesis. The students had the highest self-efficacy concerning more mechanical aspects of writing, such as using academic vocabulary and synthesising information, and the lowest self-efficacy concerning organising ideas and writing for an audience. Their beliefs could reflect immature writing development. The study’s results highlight the need for improved thesis preparation and clearer writing curriculum goals for medical students.