Ecological momentary assessment (EMA) holds significant potential within aggression research. It affords researchers the possibility of collecting data in ecological context, in near real time. However, there is a lack of measures of aggression that have been developed and validated for use in EMA contexts. In this study, we report on the validation of a measure specifically designed to address this need: the Aggression-ES-A. Building on a previous pilot study, we evaluate the within- and between-person reliability, nomological net and associations with a validated trait measure of aggression of the Aggression-ES-A in a sample of N = 255 emerging adults from the Zurich Project on Social Development from Childhood to Adulthood (z-proso). Using multilevel confirmatory factor analysis and structural equation modeling, we found support for the factorial validity, reliability, and concurrent validity of the Aggression-ES-A scores. Results support the use of the Aggression-ES-A in EMA studies utilizing community-ascertained samples.