This study aims to develop and validate a comprehensive scale for assessing students' sustainability competence in education for sustainable development (ESD). Existing ESD assessment tools have faced criticism for lacking rigorous design and validation. Following a three-phase procedure proposed by Boateng et al., the study utilized qualitative and quantitative methods to develop and validate the scale. The scale was administered to 708 students participating in an extracurricular eTournament on Sustainable Development Goals organized by a Hong Kong university. The validation process involved expert panels, cognitive interviews, factor analyses, and assessments of internal consistency. Using Wiek et al.'s five-competence framework, 16 ESD learning outcomes were generated, forming the basis for the scale items. The survey included a mini-scenario introduction to enable students' evaluation of sustainability practices, even with limited experience. Data analysis revealed a four-factor structure that could be both distinguished and integrated under a single upper-level factor. The scale is developed with the aim to better measuring tertiary students' sustainability competence via a holistic and integrated approach, which completes the constructive alignment of current ESD, particularly in extracurricular settings.