With nearly half of all children under age six living in poverty, it is imperative to explore the development of income-related childhood aggression. Here, we specifically examine how family socioeconomic status (SES) relates to children's physical and relational aggression, and how this relation may be moderated by burgeoning social cognition. Preschool-age children (n = 89; ages 3-6 years), who were reported by teachers as behaving aggressively at least some of the time, completed theory of mind (ToM) measures, and parents provided family demographic information. Multivariate moderation analyses revealed that low-SES children earned higher scores on relational aggression compared to high-SES children. ToM moderated this relation; children from impoverished backgrounds received higher scores on relational aggression if they underperformed on ToM, but at high levels of ToM there were no differences in aggression by SES. Implications and limitations are discussed.