Primary-secondary school transitions are a critical period for children, which pose a heightened risk for the development of poor emotional wellbeing and mental health. Our ability to identify and support children during this critical juncture is limited, however, by the lack of a robust, sensitive, and standardized scale to measure children’s emotional wellbeing in the context of primary-secondary school transitions, comprehensively and longitudinally. The Primary-Secondary School Transitions Emotional Wellbeing Scale (P-S WELLS) has been developed following a multi-informant, mixed-methods model to overcome this gap. It is qualitatively different and novel in its approach (i.e., asking children about their feelings toward the changes they are negotiating in context), and in its longitudinal design and operationalization. This registered report describes how P-S WELLS will be validated through longitudinal psychometric assessment (Study 1) and longitudinal validation (Study 2) on two discrete samples, across two transition periods.