Ibraheem Musaibli, who is now at a holding facility, was detained by the Syrian Democratic Forces (SDF) -- the U.S.-led coalition's partner force fighting ISIS in Syria, the official said.
The New York Times, which was first to report about Musaibli's capture, said he is 28 years old and from Dearborn, Michigan.
Musaibli, along with an Indiana woman whose husband joined ISIS and who is also detained, may be brought to the U.S. for prosecution, the Times reported.
The official confirmed to ABC News that the woman's name is Samantha Elhassani.
Musaibli is only the second U.S. citizen known to have been captured fighting for ISIS in Syria.
Last September, the Pentagon confirmed that a U.S. citizen had been detained after surrendering to the SDF. At first, the administration attempted to move him to Saudi Arabian custody but was blocked by a U.S. federal court after the ACLU sued on his behalf.
Now, the Pentagon has announced its intention to drop him back off in Syria in the same town where he was originally detained. The ACLU has sued to block this as well, and his case is awaiting a decision in federal court.
While in custody, he has never been charged with a crime.