<p>Wetland conservation is a potentially effective but undervalued strategy in managing growing flood risk under climate change. Here we address this challenge by quantifying the effects of wetlands in reducing riverine flood losses across the USA using flood insurance claims data and annually observed changes in wetland area between 1985 and 2023. We find that claim amounts for individual properties increase by 0.01–0.03% per hectare of upstream wetland loss. Based on these effects, we estimate that wetland loss since 1985 has increased flood insurance claims by a total of US$10.1 billion or 9.0%. We also calculate the marginal value of wetlands in reducing riverine flood losses for each subwatershed in the USA. In 16% of subwatersheds, the marginal benefits of flood mitigation outweigh the marginal costs of land conservation. Policymakers can use this information for conducting regulatory impact assessments, designing land-use policies and pricing flood insurance.</p>