PSA from the Federal Trade Commission:

The Federal Trade Commission has a new tool for victims of identity theft. Now, people can report tax-related identity theft directly to the IRS online at IdentityTheft.gov.

Tax-related identity theft happens when someone steals your Social Security number, files a tax return, and claims your refund. You might find out about it when you try to e-file — and learn that someone already submitted a return — or when the IRS sends you a letter saying it received a suspicious tax return with your Social Security number. If that happens, you need to file an IRS Identity Theft Affidavit, which you can do at IdentityTheft.gov.

IdentityTheft.gov (Robodeidentidad.gov in Spanish) is secure and free to use. This blog post tells more about how the site works.