Despite an emerging emphasis on the role of engineers in achieving the SDGs, there is still a lack of agreement on which competencies should be prioritised to prepare them to tackle future sustainability challenges. By formulating a theoretical framework of key sustainable competencies based on prior studies, this study investigated engineering students' subjective views on which competencies they considered as the most important to support them in becoming sustainable engineers/designers. Using Q methodology, views were collected from 26 master students in a Sustainable Design programme in a Danish systemic problem- and project-based (PBL) context. Three key competencies were identified: collaboration, interdisciplinarity, and systemic thinking. Competences within the 'doing' and 'being' dimensions were represented the strongest. There is a hierarchy between the competencies, with collaboration being the highest level, as it demonstrates that it is transversal to the others. These results emphasised the role of PBL in developing competencies for sustainability and in reinforcing the value of Education for Sustainable Development.