This paper investigates how historically intensive irrigation systems influenced enduring institutional and cultural traits that constrain firms’ access to finance. Combining geo-climatic measures of irrigation potential with firm-level data from 174 ethnic regions across 146 countries, we find that historically irrigated societies are characterised by weaker property rights, lower trust in financial institutions, and greater reliance on internal financing. Firms in these regions report more severe financial obstacles and higher rejection rates from banks. Implementing a spatial regression discontinuity design around the Lower Rhine and using irrigation potential as an instrument, we provide evidence consistent with a long-term influence of historical irrigation on modern credit frictions. The effects are most evident among privately owned domestic firms, unaffiliated firms, and those with higher female ownership. These findings indicate that ancient irrigation infrastructure is associated with persistent imprints on contemporary financial markets.