In research on aggression, implicit association tests (IATs) have been constructed to elucidate automatic processes involved in aggressiveness. Despite an increasing number of applications of the “Aggressiveness‐IAT” in basic and applied research, the psychometric properties of this method for measuring an automatic aggressive self‐concept have not been comprehensively investigated. Although the Aggressiveness‐IAT has been used both as a cross‐situationally consistent trait measure and as a measure to indicate situational changes, prior studies have not tested to what extent it reliably captures a stable trait vs. an occasion‐specific aggressive self‐concept. The present research scrutinizes the psychometric properties of the Aggressiveness‐IAT by addressing two issues. First, we tested the reliability, consistency, and occasion specificity of the Aggressiveness‐IAT in a longitudinal panel study with four waves and 574 Austrian school children/adolescents by applying latent‐state trait (LST) theory. Second, we validated latent trait scores of the IAT vis‐à‐vis other measures either clearly related to aggression or not. Results demonstrate that 20–30% of the variance in children's and adolescents' IAT scores is situation‐unspecific (i.e., “stable”), whereas 36–50% are situation‐specific. Regarding its construct validity, the Aggressiveness‐IAT is correlated with explicit measures of aggression and related constructs, but it is not associated with discriminant variables (e.g., school achievement). Implications for using the Aggressiveness‐IAT are discussed in the light of these findings. Aggr. Behav. 9999:1–12, 2014. © 2014 Wiley Periodicals, Inc.