What factors lead to greater levels of generalized trust in society? The research field has established four channels through which it is commonly argued trust is affected at the macro level—economic inequality, civic participation, ethnic heterogeneity, and institutional quality. However, the quanti-tative field has focused overwhelmingly at the national level of analysis, mostly ignoring sub-national variation in trust, or focusing on single countries. We fill that gap in this study, which of-fers the largest and most comprehensive sub-national assessment of this salient question to date, including 206 regions in 24 European countries using data from an original survey of over 85,000 respondents conducted in 2013. Upon mapping national and regional levels of social trust based on our novel data, we find strong variation in levels of generalized trust within many countries, a phe-nomenon that has been often overlooked by national-level studies. We use a number of statistical estimations to assess the strength of the four channels through which trust can be enhanced or eroded. We find strong and robust evidence that institutional quality—measured as a public sector that allocates services impartially and without corruption—is the strongest determinant of regional variations in trust within countries, while economic inequality, civic participation and ethnic hetero-geneity are not significant factors in explaining variations in trust patterns.