"The FBI verified the identity of a body found in Iraq this morning. While additional forensics will be completed in the United States, they believe this is the body of Tom Fox," spokesman Noel Clay said.
Mr Clay said the US government offered its "heartfelt condolences" to Mr Fox’s family when they were notified of the discovery.
But there was no news of the other three hostages, who appeared without Fox earlier this week in a video shown on Al-Jazeera television.
The government has not released details of where Mr Fox’s body had been found in the country, but Mr Clay said the White House called for the "unconditional release of all hostages" held in Iraq.
Mr Fox, a 54-year-old American from the US state of Virginia, was the father of two children and had traveled to Iraq with other Christian Peacemaker Team activists.
The four disappeared on November 26 and shortly after a group calling itself Brigades of the Swords of Righteousness threatened to kill them unless all Iraqi prisoners were released, according to a video message broadcast on Arab television.
Another video broadcast on Al-Jazeera television on March 7 showed footage of Mr Fox's other abducted and still-missing colleagues, Canadians Harmeet Sooden, 32, and James Loney, 41, and Briton Norman Kember, 74.
However, Mr Fox did not appear in the video.