I examine the effect of mandatory climate risk disclosures on insurers' underwriting business by exploiting the U.S. insurance industry's adoption of the Climate Risk Disclosure Survey (CRDS), which mandates that certain insurers respond to questions about their climate risk strategies. Using a staggered difference-in-differences research design, I find that insurers affected by the mandate experience an increase in their underwriting business after CRDS adoption relative to unaffected insurers. This outcome suggests that the CRDS mandate results in the affected insurers gaining a competitive advantage in their underwriting business. This positive effect is more pronounced when insurers' underwriting business is more exposed to customers concerned about climate risk, the insurers experience more underwriting business competition, or their headquarters are in a Democratic state. I also perform a textual analysis of the affected insurers' CRDS responses, which offers supplementary evidence of a positive association between insurers' proactiveness in addressing climate risk issues and their future underwriting business. My study offers the novel insight that by encouraging firms to develop business strategies that take climate risk into account and communicating such strategies publicly, mandated climate risk disclosure can enhance firms' competitive advantage.