There is an ongoing debate on the relationship between Islam and (lack of) democracy. Considerable literature shows that Islam, represented as an informal institution by Muslim population share, has a negative effect on democracy. This study examines the effects of formal institutions, specifically constitutions that prescribe Islamic law ( Shari’a) as a source of legislation, on democracy. We use a newly developed coding of the degree to which Islam is incorporated in constitutions. Our empirical results show that the constitutional entrenchment of Islamic law has a negative and significant effect on democracy. Our findings are robust to using different estimators and instrumental variable regressions, employing alternative measures of democracy and controlling for Muslim population, natural resource wealth, and additional control variables. While we show that Islamic constitutionalism is a reason for a democracy deficit in Muslim-majority countries, we find no evidence that Islam is inimical to democracy when not entrenched in the constitution.