A recent literature explores how domestic institutions affect politicians’ incentives to enter into international agreements (IAs). We contribute to this field by systematically testing the impact of a broad set of domestic institutional design features. This allows us to compare new and established political economy explanations of IA entry. For this purpose, 99 democracies are analyzed over the period 1975–2010. We find that domestic institutions determine countries’ disposition to enter into IAs, as predicted by political economic theory. For example, democracies with majoritarian electoral institutions are less likely to conclude IAs than other democracies. Countries also conclude more IAs when their democratic institutions are long-lived and they lack an independent judiciary. However, programmatic parties and the number of domestic veto players are not associated with IA-making. The key take-away of this study is that specific domestic institutions matter for how frequently states make formal deals with each other.