Common notions about the source of communal land conflict in Africa have long explained it as growing out of conditions of environmental scarcity. This article argues instead that the institutional structure of the legal system is central to understanding which countries are prone to experience communal land conflict. When competing customary and modern jurisdictions coexist in countries inhabited by mixed identity groups, the conflicting sources of legal authority lead to insecurity about which source of law will prevail. Because the source of law is contested, conflict parties cannot trust the legal system to predictably adjudicate disputes, which encourages the use of extrajudicial vigilante measures. Using new data on communal violence in West Africa, this argument is examined for the period 1990–2009. The results show that in countries where competing jurisdictions exist, communal land conflict is 200–350% more likely. These findings suggest that researchers should consider the role of legal institutions and processes in relation to social unrest and collective violence.