Chávez, who faces an election on 7 October, suggested the man is part of a plot to destabilise the country if he is re-elected.
Chávez said the Hispanic man was detained on 4 August while crossing into Venezuela from Colombia. The president said the man was carrying a US passport with entrance and exit stamps from countries including Iraq, Afghanistan and Libya as well as a notebook containing geographical co-ordinates.
The man's identity was not released. Chávez did not say where he was being interrogated.
An official from the US embassy in Caracas did not answer repeated telephone calls seeking comment on Chávez's announcement.
"He has all the appearance of a mercenary," Chávez said, speaking during a campaign rally in the coastal state of Vargas. "We are interrogating him."
The man tore up part of the notebook in his possession when he was detained, Chávez said.
Chávez, who often makes sweeping and bellicose denouncements of his opponents, suggested - without offering evidence - that the American might have been recruited by government opponents to instigate violent protests if opposition presidential candidate Henrique Capriles loses the election. Chávez has repeatedly vowed to win re-election and continue trying to steer Venezuela toward socialism.
Comment: Chavez's "bellicose statements" have the advantage of being far more objective than the propaganda used against the new Venezuela he has created. He also has every reason to believe the US will do what they did against him in 2002 and in Syria and Iran since then: SYRIA: Venezuela 2002 faked "Revolution" Deja-Vu
The president has repeatedly claimed the opposition plans to accuse election officials of rigging the vote and refuse to accept the results if he is victorious, an allegation that Capriles and fellow opposition leaders deny.
"A group of the bourgeoisie is preparing to reject the people's triumph, that's very clear," Chávez told the crowd of cheering supporters.
Anti-Chávez politicians also reject the president's allegations they are trying to stir up trouble by campaigning in areas that have been bastions of support for Chávez or conspiring with US officials to provoke upheaval if Capriles fails to defeat the incumbent who is trying to win a fresh six-year term.
So far campaigning ahead of the presidential vote has mostly been peaceful, but observers say there is a danger that Venezuela's deep political polarisation and rising tensions between allies and adversaries of Chávez could boil over.