Conditionally Accepted for publication in International Studies Quarterly What explains variation in sovereign borrowing costs, and what role does information about the state of a country's financial sector play in the determination of those costs? Investors charge more interest when there are higher default risks. When estimating default risks, investors consider more than explicit public debt levels and include the risk that the financial sector poses to sovereigns in their calculations. Investors are more confident of their assessments when regulators release credible data on the shape and health of their financial sectors. Investors reward more transparent governments with lower sovereign borrowing costs. At the same time, we predict that the e↵ectiveness of transparency declines as public debt increases. Testing this argument requires a measure of transparency, so we create a new Financial Data Transparency (FDT) Index. The Index measures governments' willingness to release credible financial system data. Using the FDT and a sample of high-income OECD countries, we find that such transparency reduces sovereign borrowing costs. The e↵ects are conditional on the level of public indebtedness. Transparent countries with low debt have lower and less volatile borrowing costs.