The number of children experiencing poor emotional wellbeing, which can lead to clinically significant mental health conditions in the long term, is increasing rapidly, as are government initiatives outlining the 'frontline role' of the school in supporting children's emotional wellbeing during critical periods such as primary-secondary school transitions. However, both concepts ('primary-secondary school transitions' and 'emotional wellbeing') are poorly and inconsistently conceptualised and/or theoretically defined. This has significant consequences for identifying and supporting children's emotional wellbeing. The aim of this concept analysis is to report a synthesis of the extant literature and define emotional wellbeing in the context of primary-secondary school transitions as a concept of emerging importance. The Walker and Avant (2005) method was utilised as a framework. Attributes, antecedents, and empirical referents were identified through synthesis of methodological approaches (specifically an international systematic literature review, UK-wide survey and 10 focus groups) and the mapping of multiple stakeholder perspectives (specifically researchers, educational practitioners, policy influencers and/or makers, and children). Attributes of emotional wellbeing in the context of primary-secondary school transitions include children's affective experience of navigating primary-secondary school transitions in the here-and-now (e.g. presence of both positive and negative emotions), and their evaluations of their emotional wellbeing both globally and in the context of specific domains. Antecedents include children's perceptions of their internal and external resources to manage the demands of primary-secondary school transitions and maintain a stable affective state. Consequences (positive and negative) include academic attainment, social adjustment, and school belonging in the short-term, and mental health, life chances, and social inequalities in the long-term. Our novel conceptualisation overcomes limitations in existing understanding of both primary-secondary school transitions and emotional wellbeing, establishing a foundation for developing a more cohesive and theoretical body of work within the field. Our conceptualisation and operational definition will have notable positive implications in advancing future research, policy, and practice, which are outlined.