Predictors of the physical and relational aggressive behavior of children in foster care were examined (N = 160, 50.9% male, M age = 7.57, SD = 2.39). First, predictors representative of children’s placement histories were examined in relation to the children’s aggression at T1. Next, predictors representing characteristics of the current family environment were examined in relation to the children’s aggression at T2 (four months later). Results revealed that a greater number of prior group home placements and being in a non-kinship home were associated with higher physical aggression at T1. A greater number of prior group home placements, a fewer number of regular home placements, being in a non-kinship home, and prior removal from the home due to neglect were associated with higher relational aggression at T1. The results also revealed that higher foster sibling relational aggression at T1 predicted lower child physical aggression at T2. If foster siblings were biological children of the foster parent, higher levels of a foster sibling’s physical aggression at T1 predicted reduced child physical aggression at T2. The opposite pattern was observed if foster siblings were not biological children of the foster parent. Lastly, longer time in the current placement, more children in the home, and the presence of a sibling that was a biological child of the parent predicted higher child relational aggression at T2. These findings provide initial insights into how placement history and current family environment are associated with the physical and relational aggressive behavior of children in foster care.