The US director of national intelligence, James Clapper, is in New Zealand for farewell talks with the Government and spy chiefs.
It had been a secret visit until Mr Key was asked at his post cabinet press conference if he knew why a Globemaster C-17 was parked at Wellington airport.
Mr Key said "Yes, because Jim Clapper's in town."
Mr Clapper is the chief intelligence adviser to Mr Obama and oversees the 16 agencies within the US intelligence community including the CIA and National Security Agency.
Mr Key said Mr Clapper, who has been America's intelligence chief since 2010, was on his way to a Five Eyes meeting in Australia.