A raft of new research on the causes and effects of political corruption has emerged in recent years, in tandem with a separate, growing focus on the effects of political institutions on important outcomes such as economic growth, social equality and political stability. Yet we know little about the possible role of different political institutional arrangements on political corruption. This article examines the impact of territorial sovereignty (unitary or federal) and the composition of the executive (parliamentary or presidential) on levels of perceived political corruption cross-nationally. We find that unitary and parliamentary forms of government help reduce levels of corruption. To explain this result, we explore a series of seven potential causal mechanisms that emerge out of the competing centralist and decentralist theoretical paradigms: (1) openness, transparency and information costs, (2) intergovernmental competition, (3) localism, (4) party competition, (5) decision rules, (6) collective action problems, and (7) public administration. Our empirical findings and our analysis of causal mechanisms suggest that centralized constitutions help foster lower levels of political corruption.