The embassy could not immediately provide further information about where the letter came from or what was suspicious about it.
A spokesman for France's judicial police said it had deployed a mobile laboratory to test for poisonous substances at the embassy, which lies close to the historic Champs-Elysees in the centre of the French capital.
Mailrooms at US diplomatic facilities worldwide are always on the lookout for suspicious packages, amid fears that bombs or toxic materials could be sent via the post.
Suspicious mail became a greater security focus after five people in the United States were killed and 17 fell ill after opening letters containing anthrax in 2001.
Postal facilities nationwide were shut for inspection after the letters containing anthrax spores were sent to politicians and news organisations in the aftermath of the September 11, 2001 attacks.
The FBI concluded that an army scientist, Bruce Ivins, was responsible for the attacks.
Ivins, who killed himself in 2008, denied involvement, and his family and some friends have continued to insist he was innocent.