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.
"He is retiring before President [Barack] Obama effectively steps down in 2017 so I don't know if he'll be back in New Zealand again."
Mr Key said he had met him on a couple of occasions and he had "great insights into intelligence and what is happening around the world."
Mr Clapper's visit comes hard on the heels of the release last week of a report by Sir Michael Cullen and Dame Patsy Reddy recommending significant changes to the way New Zealand's spy agencies operate.
Mr Key said he would be discussing the report with Mr Clapper.
The report recommends that the Government Communications and Security Bureau (GCSB), the foreign intelligence agency, be given the power to spy on New Zealanders under its own warrant, a significant expansion of its current powers.
At present it is limited to using its high-tech interception capabilities only to prevent or deal with cyber security threats, helping other agencies under their warrants, or when a New Zealander is acting as the agent of a foreign power.
Mr Clapper has made at least one other previous visit to New Zealand, in March 2011 when he met with Prime Minister John Key and spy agency chiefs.
It was revealed earlier this year that Mr Clapper's personal emails had been hacked by a teenager.
Mr Clapper's plane is sitting on the military side of the Wellington airport tarmac.
Across the other side is the Airbus 340 used to fly the Iranian foreign minister, Mohammad Javad Zarif on a visit to New Zealand.
He too is heading to Australia after his visit.