The fighter gave himself up when he was approached near the town of Sinjar on Monday morning, an Iraqi Kurdish general said.
He was reportedly carrying a large quantity of cash, three phones and a US driving licence.
The US is leading an international military coalition against IS.
The US state department said that it was aware of reports that a US citizen had been captured by Kurdish forces in northern Iraq, Reuters reports.
"We are in touch with Iraqi and Kurdish authorities to determine the veracity of these reports," a state department official said.
CBS News said the man was trying to enter Turkey when he was intercepted by Kurdish forces.
Maj Gen Feisal Helkani of the Kurdish peshmerga forces said the man was currently being held by the peshmerga for interrogation, AP reports.

The investors, Filmyard, includes construction magnate Ron Tutor and investment firm Colony Capital.
Disney has been negotiating with potential buyers for months to sell the division, which has made films such as Pulp Fiction and Shakespeare in Love.
Earlier this week, Disney bought the social gaming developer Playdom.
"Although we are very proud of Miramax's many accomplishments, our current strategy for Walt Disney Studios is to focus on the development of great motion pictures under the Disney, Pixar and Marvel brands," Disney's chief executive Robert Iger said.
Disney wants to concentrate on these family-orientated divisions, which produce popular films such as Toy Story, that also come with highly profitable merchandising opportunities.
"Disney has been very smart with its film strategy," media analyst Theresa Wise told BBC World Business Report.
"One of the big issues with movies is very few films break even... so they need other revenues. Disney has the fantastic ability to sell family films and all the merchandise that goes with it.
Disney has owned Miramax since 1993 when it bought the firm from Hollywood heavyweights Harvey and Bob Weinstein.
The Weinstein brothers, who recently attempted to buy it back from Disney, named the film company after their parents Miriam and Max.

The body of James Chadwick, 37, was found at an address in Holland Street last Wednesday.
At Aberdeen Sheriff Court, Ryan Gibb and Cameron Laurie, both 30, were charged with murder and attempting to defeat the ends of justice.
They made no plea and were remanded in custody. They are due to appear in court again next week.

The victim was found with serious injuries at a house in Unsworth Way in Oldham, Greater Manchester, at about 19:50 GMT on Thursday.
He was taken to hospital and died a short time later. Details of his injuries have not yet been revealed.
The house was cordoned off while police carried out an investigation. A 58-year-old woman has been taken into custody for questioning.

Officers described the incident near Blairhill Station, in Coatbridge, North Lanarkshire, on Sunday night as "stupid and dangerous".
The shopping trolley was hurled through overhead power lines which could have electrocuted the person.
A train hit the trolley, but there was no serious damage to the vehicle or any injuries on board.
British Transport Police have appealed for anyone with information to contact them.
PC Mark Harrington said: "We believe the trolley was thrown onto the tracks before the train struck it.
"It's incredible that whoever threw it was not electrocuted, as the trolley has gone between the overhead wires.
"This was a stupid and dangerous thing to do and could have had very serious consequences, not only for the person throwing the trolley but also for the driver and all the passengers of the train that collided with it.
"This happened in a residential area so it is likely that someone will have seen or heard people by the tracks around the time of the incident.
"I would like to speak to anyone who has any information that could help with this investigation."
US officials are checking reports that an American member of the so-called "Islamic State" group (IS) has been held in northern Iraq.
[NEXT_CONCEPT]
Entertainment giant Walt Disney has announced the sale of its Miramax film division for about $660m to a group of private equity investors.
[NEXT_CONCEPT]
Two men have appeared in court charged with the murder of a man in Aberdeen.
[NEXT_CONCEPT]
A woman has been arrested on suspicion of murdering a 56-year-old man.
[NEXT_CONCEPT]
A person who threw a trolley onto train tracks is lucky to be alive, transport police have said.