He later said he believed Trump could be talked around, saying Trump had a business brain and would eventually see its benefits.
"We might have to be a little bit creative to work out how to get the US there."
As well as a TPP leaders' meeting, Key had a chance to informally sound out fellow leaders on the sidelines, including Japan's Shinzo Abe, US President Barack Obama, Canadian President Justin Trudeau and Mexico's Enrique Pena Nieto. He also met Australia's Prime Minister Malcolm Turnbull for a drink on Sunday night.
Key also spoke to China's President Xi Jinping, who has been promoting the China-led Regional Comprehensive Economic Partnership as an alternative to TPP.
Key's day had begun with him speaking at a CEO's Summit alongside Mexico's Prime Minister Enrique Pena Nieto. At that he urged the other leaders to hold their nerve on free trade and pushed home the message that the US would lose influence to China if it withdrew.
He later pointed to trade and climate change as areas China could be seen to be offering stronger leadership than the USA if Trump put his campaign promises into practice.
Key said after the US election, New Zealand trade negotiators had estimated what the benefits in the deal would be without the US - it was expected to be about two thirds of the $2.7 billion a year with the US.
Key will leave to return to New Zealand on Monday afternoon after the leaders' retreat for the 21 Apec leaders.