"I personally think President Trump will be very much like chairman of the corporation Trump is. In the end, he's a flexible business guy, he's going to realise he's got a role to play, he's going to realise the US has shown global leadership for a long time and I think he's going to get into office and do that."
He said one possibility was to make a cosmetic change that would allow Trump to say he could sign up without losing face - Key joked that could be a name change to the "Trump Pacific Partnership."
Key made the comments just before heading into a meeting with the other TPP leaders, including US President Barack Obama, who are now debating whether to stick to the agreement if the US does drop out as Trump has signalled.
Key said the Apec countries should not fall into "tremendous despair" because of Trump's election and completely give up.
"Even if the United States does not want to engage in free trade, President Trump has to know others do, and they are going to."
He said the TPP leaders had to send a strong message that free trade worked, and they believed in it, and would go ahead with or without Trump.
Key further ramped up his warnings that the US would be out in the cold.
"There needs to be a bit of reality, because the reason President Obama did TPP was all about the Asia pivot, it was all about the United States showing leadership in the Asia region."
He said the Pentagon and Defense Department had argued United States needed to be a leader in that region.
"We really like the US being in the region. We think they are great partners, great friends and we think they add something to all the countries there. But in the end if the US is not there, that void has to be filled.
And it will be filled by China. And we have a good relationship with China, it's a matter for the US whether they want to be there or not.
Key was taking part in the panel with Mexico President Enrique Pena Nieto - the leader of another TPP country for whom Trump's election also means a renegotiation of the long standing NAFTA agreement between Mexico, the USA and Canada.
Nieto told the summit that Mexico, like other parts of the world, would have to start a new relationship with the USA.
He too voiced support for free trade.
"Let us not be confounded, let us not be confused by these protectionist sentiments that are now growing in parts of the world."
However, Nieto said he was "optimistic" that NAFTA could be re-negotiated and "modernised."
It is the first of Key's two-day visit to Peru for the Apec Summit and began with bilateral meetings with fellow TPP leaders Chile's President Michelle Bachelet and Peru President Pedro Pablo Kuczynskil.
Tomorrow the leaders of the 21 economies in Apec will go into the leaders 'retreat'.
US President Barack Obama is at the conference for his final summit before handing over to Trump in January.