The show boss revealed the news when he arrived at the Galen Centre in Los Angeles for auditions on the US version of the singing competition.
According to various reports, Cowell said: "Gary Barlow is the first person confirmed."
He apparently refused to confirm that Barlow would replace Cheryl Cole, who last week was added to the panel of the US show.
There has been no official confirmation of Cowell's statement.
Barlow was reported to have finalised a deal over the weekend.
It was suggested that Dannii Minogue has not yet signed a deal to return to the programme, but that Louis Walsh will definitely be back.