Background: Trait emotional intelligence ("trait EI" or "trait emotional self-efficacy") is a constellation of emotion-related self-perceptions and dispositions located at the lower levels of personality hierarchies. This paper examines the validity of this construct, as operationalized by the Trait Emotional Intelligence Questionnaire-Child Form (TEIQue-CF), in primary schoolchildren. Aims: The main aim was to examine the construct validity of trait EI in middle and late childhood by exploring its relationships with cognitive ability, emotion perception, and social behaviour. Sample: The sample comprised 140 children aged between 8 and 12 years (M = 9.26 years, SD = 1.00 year; 63 girls) from two English state primary schools. Method: Pupils completed the TEIQue-CF, the standard progressive matrices (SPM), the guess who peer assessment, the social skills training (SST) test, and the assessment of children's emotion skills (ACES) during formal class periods. The procedure took approximately two hours with a short break between assessments. Results: Trait EI scores were positively related both to peer-rated prosocial behaviour and to overall peer competence. They also predicted emotion perception accuracy beyond overall peer competence. As hypothesized in trait EI theory, the construct was unrelated to IQ (Raven's matrices) and academic performance. Conclusions: Trait EI is successfully operationalized through the TEIQue-CF and has important and multifaceted implications for the socialization of primary schoolchildren.