Purpose: For professional sustainability work, graduates need to be able to work in teams and collaborate with stakeholders; in other words, they need to have developed interpersonal competence. There is growing evidence that project-based sustainability courses facilitate interpersonal competence development. However, research so far has focused on single case studies and on assessing learning outcomes. The purpose of this study is to deepen the understanding of how graduate students learn interpersonal competence in project-based sustainability courses. Design/methodology/approach: This study adopts a multi-case study approach triangulating observations, semi-structured interviews and focus groups supported by Photovoice method. A comparison of three project-based sustainability courses in graduate programs at universities in the USA, Germany, Switzerland and Spain is conducted to gain generalizable insights on how interpersonal competence can be developed through project-based sustainability courses. Findings: Receiving inputs, experiencing, reflecting and experimenting are four learning processes supportive of interpersonal competence development. Interpersonal attitudes seem to be mostly learned through a combination of experiencing and reflecting, followed by experimenting; not surprisingly, interpersonal knowledge is mostly developed through a combination of receiving inputs, experiencing and (collective) reflection; and interpersonal skills seem to be mostly learned through a combination of receiving inputs and experimenting, or, more directly, experiencing and experimenting. Practical implications: These findings support the unique learning opportunities offered through project-based sustainability courses and can help instructors to better facilitate students' development of interpersonal competence. Originality/value: The value of this study is three-fold--(i) it provides a comprehensive picture of interpersonal competence, including attitudes, knowledge, and skills; (ii) it spells out specific teaching and learning processes; and (iii) it links these to specific interpersonal competence facets and components.