“It means a lot of trade, a tremendous amount of trade. They want to do business with us so badly,” Mr Trump told journalists late on Friday.
Earlier, the president had tweeted: “Congratulations to Boris Johnson on his great WIN! This deal has the potential to be far bigger and more lucrative than any deal that could be made with the EU. Celebrate Boris!”
Both the US president and Mr Johnson have repeatedly spoken of their desire for a deal between the two countries amid concerns parts of the NHS could be made available to US markets by a Conservative government.
Ahead of the vote, Labour politicians maintained a Tory win risked opening up parts of the NHS to US companies, but both the Conservatives and the White House denied this was the case.
Mr Trump said the NHS was “nothing to do” with his recent visit to the UK, but had previously said: “When you’re dealing in trade, everything is on the table.”
The US president said with Mr Johnson’s parliamentary majority, the UK and US were “now free to strike a massive new trade deal after Brexit,” which Mr Johnson insists will happen at the end of January.
Speaking about the election and the UK, Mr Trump said: “I want to congratulate Boris Johnson on a terrific victory. I think that might be a harbinger of what’s to come in our country – it was last time.
“I’m sure people will be thrilled to hear that. But a lot of people will be actually, a very big percentage of people, because this was a tremendous victory last night. And it’s very interesting, the final votes are being tallied right now, but the numbers are tremendous.
“So I want to congratulate, he’s a friend of mine, it’s going to be a great thing for the United States also, because it means a lot of trade, a tremendous amount of trade. They want to do business with us so badly. Under the European it was very, very hard for them to do business with us.”
He added: “We just made our big deal, as you know, with Mexico, Canada, we have a tremendous trade deal that’s going through the house now. It’s going to be obviously approved and it’s tremendous for our country. It’s really tremendous for the region but it’s fantastic for the US.”
Despite reservations Mr Johnson has previously voiced about Mr Trump, including saying the only reason he wouldn’t visit some parts of New York “is the real risk of meeting Donald Trump”, the US president has been a strong supporter of the UK prime minister.
Last month, Mr Trump said a victory for Jeremy Corbyn would be “so bad” for Britain and that Mr Johnson should form a pact with Brexit Party leader Nigel Farage.
Speaking to the BBC after Mr Johnson’s victory speech at Downing Street, the US ambassador to the UK, Woody Johnson said: “As far as this president’s concerned, [the UK] has never been anything but the front of the queue” for a trade deal.
The ambassador said Mr Trump’s congratulatory tweet reiterated that the president now wants a “free, fair and balanced” trade deal and “he wants to get it done quickly”.