21st Century Endangered Species Transparency Act This bill amends the Endangered Species Act of 1973 to require the Department of the Interior or the Department of Commerce, as appropriate, to make publicly available on the Internet the best scientific and commercial data available that are the basis for the determination of whether a species is an endangered species or a threatened species, including each proposed regulation for the listing of a species. The departments are prohibited from making the information publicly available when: (1) the public disclosure of the information is prohibited by state law relating to the protection of personal information, and (2) the state makes a request to a department to withhold the information.