Do democracies and autocracies differ when it comes to whether and how they provide third-party support to warring parties in civil wars? We argue that the political institutions of potential third-party states have important consequences for both questions. We emphasize how three particular institutional characteristics of democratic polities constrain decision-makers. This makes democracies less likely than autocracies to intervene in intra-state conflicts in general, and less likely to provide combat-intensive support specifically. An empirical analysis of incidents of third-party support to actors in civil wars in the period 1975–2009 corroborates the overall argument, although the results regarding support types are less clear. These results have important implications not only for our understanding of civil wars but also for how foreign policy decisions are made across different regime contexts.