Background: The transition from primary to secondary school presents a challenging developmental milestone which often marks a decline in academic performance. Social-emotional skills are recognized as fundamental to academic success but longitudinal research is needed to determine the extent of their association over this transition period. Aim: This study sought to determine the association between self-reported social--emotional competencies of students in their final year of primary school (Year 6; age [approximately]11 years) and reading and numeracy performance in their first year of secondary school (Year 7; age [approximately]12 years). Sample: The study used a large Australian sample (n = 23,865), drawn from the New South Wales Child Development Study population cohort. Methods: The Middle Childhood Survey--Social-Emotional Learning assessment, administered during Year 6, comprises the five competencies defined by the Collaborative for Academic, Social and Emotional Learning (CASEL): Self-Awareness, Self-Management, Social Awareness, Relationship Skills and Responsible Decision-Making. These data were linked with students' Year 7 reading and numeracy scores from the standardized National Assessment Program--Literacy and Numeracy measure. Associations were examined in multi-level structural equation models which accounted for prior (Year 5) academic achievement and sociodemographic covariates. Multi-group analyses explored invariance across girls and boys. Results: Self-Awareness and Self-Management demonstrated significant and meaningful positive relationships with reading and numeracy performance. Associations with reading were invariant by sex but boys demonstrated significantly stronger associations than girls on numeracy. Conclusion: Findings suggest that bolstering primary school students' intrapersonal social-emotional competencies may safeguard their academic achievement over the transition into secondary school.