Purpose: This study aims to contribute to the professional competency approach in Education for Sustainability (ES) from the perspective of complexity and to the assessment of these competencies. Design/methodology/approach: A qualitative research process was used, which consisted of two main phases--a documentary analysis of the internationally accepted frameworks about professional competencies in ES and complexity and a case study. The context was the Learning Camp of Cistercian Monasteries (Catalonia, Spain), and two educators participated in this camp. Findings: The main outcomes are--a framework of professional competencies in ES from a complexity perspective, called CESC, and a rubric, as an assessment tool for professional competences in ES. Originality/value: Based on the results, the author's can state that this research is a contribution to the ES competency approach and it rectifies the lack of assessment tools in this area. The CESC is a theoretical contribution that facilitates educators' development of ES competencies based on complexity. The design of the rubric with a high level of applicability and transferability is a methodological contribution which facilitates the assessment of these competencies by educators, as individuals, and educational teams. Related to the case study, several guidelines are proposed regarding the need for educator training to advance in the mobilisation of competencies in ES.