Traditional assessment methods in tertiary education may not suit students’ diverse learning needs, values, and preferences. Co-designing assessment with students may engage them more effectively. This scoping review determined assessment co-design processes employed in tertiary education, evaluated the impacts on student learning outcomes and key factors contributing to effective assessment co-design. Seven databases were searched, limited to human studies in English language published between 2003–2023, with records screened by two reviewers. Data were presented in tabular form and a narrative synthesis. From 6233 records identified, 51 articles were included. Three main assessment co-design processes were: co-design of assessment rubrics and marking criteria; assessment co-design with larger student groups (e.g. via multiple-choice question pools and co-creation workshops); and intensive assessment co-design with smaller groups. Positive student outcomes included increased assessment literacy, student autonomy, engagement and interest and development of professional and learning skills. Staff support and effective communication between team members were some key enablers, while some barriers included time constraints and student resistance to participation. There were diverse co-design processes and outcomes from assessment co-design across disciplines. Future research should prioritise assessment co-design with international students for inclusion and equity and graduate students to accommodate advanced learning skills.