Financial deepening contributes to economic development, but its effect on the carbon intensity of production is an open empirical question. If banks finance investments in new, greener technologies, they can contribute to lowering carbon dioxide emissions per unit of output. But if they finance investments in more traditional, carbon-intensive technologies, they can contribute to increasing carbon dioxide emissions per unit of output. This paper studies the impact of financial deepening—an increased provision of bank credit as a share of gross domestic product—on carbon dioxide emissions per dollar of gross domestic product in a global sample of 125 economies from 1990 to 2019. Using a local projections approach, the paper finds that, on average, financial deepening leads to a relative increase in carbon dioxide emissions per dollar of gross domestic product, indicating that financial institutions finance relatively more carbon-intensive investments and consumption. However, a better institutional environment mitigates this adverse effect of financial deepening: conditional local projections reveal that in countries with more environmental regulations, a stronger rule of law, and a financial system that is relatively more market- than bank-based, financial deepening does not lead to higher carbon dioxide emissions per dollar of gross domestic product. Specifically, the results show that countries with an initially lower carbon intensity of production can mitigate the average adverse effect of financial deepening on carbon dioxide emissions per dollar of gross domestic product by improving their general institutional environment proxied by adherence to the rule of law, and, to some extent, by developing their financial markets. By contrast, countries with an initially higher carbon intensity of production are better off focusing on environmental regulations to mitigate the unconditional adverse effect of financial deepening on carbon dioxide emissions per dollar of gross domestic product.