Childhood social and emotional competencies are recognized as teachable skills affecting well-being and developmental outcomes across the life span. This study sought to develop and validate a brief self-report measure of social–emotional competencies in middle childhood. The study used items from the 2015 Middle Childhood Survey, administered to a representative subsample of the New South Wales Child Development Study cohort, comprising sixth grade students (n = 26,837; aged 11–12 years) attending primary school in New South Wales, Australia. Exploratory and confirmatory factor analyses assessed the latent structure of social–emotional competencies, and item response theory and construct validity analyses evaluated the reliability, validity, and psychometric properties of the derived measure. A correlated five-factor model outperformed other latent structures (one-factor, higher order, and bifactor models) and was consistent with the framework developed by the Collaborative for Academic, Social, and Emotional Learning that informs the Australian school-based social–emotional learning curriculum, incorporating the following: Self-Awareness; Self-Management; Social Awareness; Relationship Skills; and Responsible Decision-Making. This brief (20-item), psychometrically sound, self-report measure of social–emotional competencies in middle childhood provides capacity for exploration of these skills as mediators and moderators of developmental outcomes across the life span.