Within any type of system, the actors in the system inevitably compete over resources. With competition comes the possibility of conflict. To minimize such effects, actors often will partition the system into geographic territories. It is against the larger ecological backdrop of competition and conflict that we examine territory formation among urban street gangs. Although previous studies have examined the social and built environment where gangs form, and how the presence of a gang influences local levels of violence, we know little about how competitive interactions are tied to the formation and maintenance of gang territories. We use formal spatial Lotka–Volterra competition models to derive hypotheses about competition-driven territory formation. By using data on 563 between-gang shootings, involving 13 rival street gangs in the Hollenbeck Policing Division of Los Angeles, we show that violence strongly clusters along the boundaries between gangs in a way that is quantitatively predicted by the theory. The results suggest that even weak competitive interactions between gangs are sufficient to drive gang territory formation without recourse to other processes or assumptions.