This study examined whether exposure to changes in peer aggression predicted changes in child physical aggression (PA) in preschool children attending Norwegian Early Childhood Education and Care (ECEC) centers. Data from the Behavior Outlook Norwegian Developmental Study were used, including 956 children. In fixed effects models, within-child changes in exposure to peer aggression predicted changes in teacher-rated child PA across ages 2, 3, and 4. Moreover, changes in exposure to a peer group with two or more externalizing children increased teacher-rated child PA over time, but only for boys. No significant peer effects on parent-rated child PA were found. Findings point to the importance of avoiding the congregation of several problematic children, particularly boys, in the same ECEC groups.