This study investigates the relation between customer concentration and a supplier’s cost of equity capital. We hypothesize that a more concentrated customer base increases a supplier’s risk, which results in a higher cost of equity. Our results show a positive association between customer concentration and a supplier’s cost of equity, and this relation is more pronounced for suppliers that are more likely to lose major customers or that are more prone to larger losses if they lose such customers. Further, results from a propensity score matched sample analysis and instrumental variables regressions imply that our findings are robust to accounting for endogeneity. We also provide evidence that a supplier with a concentrated base of safer government customers has a lower cost of equity. Finally, we document a positive relation between corporate customer concentration and a supplier’s cost of debt. Overall, our findings suggest that the composition and concentration of a supplier’s customer base significantly impact its financing costs.