The man, whose name was not released, is part of the management staff in the umbrella Palestinian Liberation Organisation's (PLO) negotiations department, led by chief negotiator Saeb Ereka, and is accused of "collaborating with Israel" and leaking documents to Israel, a security official told Reuters.
Another official said the suspect had been arrested two weeks ago.
Al Ayyam newspaper described the suspect as a 20-year veteran of the PLO team and said he had confessed to the charge.
Erekat and his office declined comment.
Erekat, who also serves as the secretary general of the PLO Executive Committee, is one of the most prominent Palestinian public officials and the discovery of a mole in his office could hurt his standing.
How long he had spied for Israel and what damage he might have done remained unclear, Al Ayyam said, citing an unnamed senior Palestinian official.
Collaborating with Israel is considered a serious crime in Palestinian society. Some have been killed for doing so.
Israeli officials did not immediately respond.