Donald Trump has heaped praise on Boris Johnson, calling him "tough and smart".
The US president claimed that people were calling Mr Johnson "Britain Trump" because of their similarity.
Addressing a crowd of young US conservatives, he said Mr Johnson was "what they need" and "he'll get it done".
The president said: "We have a really good man who is going to be the prime minister of the UK now: Boris Johnson.
"Good man. He's tough and he's smart. They're saying 'Britain Trump'. They call him Britain Trump and it's people saying that's a good thing.
"They like me over there. That's what they wanted. That's what they need."
Mr Trump also suggested that Brexit Party leader Nigel Farage would "work well with Boris" to do "some tremendous things".
The new Tory leader, eager to strengthen the transatlantic relationship in anticipation of a trade deal, will hope the US President does not dwell on his past barbs.
These include accusing Mr Trump of "stupefying ignorance", and saying he was "out of his mind" and "unfit" to lead the States.
And while the deeply-desired deal is one Mr Johnson will champion, dealing with the US president may also be one of his greatest challenges after Brexit, as repeatedly learned by Theresa May.
It took a matter of minutes for the US President to use his favourite medium to celebrate Mr Johnson's victory on Tuesday.
"Congratulations to Boris Johnson on becoming the new Prime Minister of the United Kingdom. He will be great!" he tweeted.
Mr Johnson has not always been so positive about the US President.
As London mayor in 2015, he described Mr Trump as "clearly out of his mind" and "unfit" for the White House when the then-presidential candidate called for a ban on Muslims entering the US.
"What he's doing is playing the game of the terrorists and those who seek to divide us," Mr Johnson continued.
"When Donald Trump says there are parts of London that are no-go areas, I think he is betraying a quite stupefying ignorance that makes him frankly unfit to hold the office of the President of the United States."
Mr Johnson joked: "I would invite him to come and see the whole of London and take him round the city, except that I wouldn't want to expose Londoners to any unnecessary risk of meeting Donald Trump."
Despite Mr Trump's claim about the "Britain Trump" nickname, critics have suggested the similarities between the two include a proclivity for falsehoods and bombastic language.
Back in March 2016, Manhattan-born Mr Johnson described being mistaken for his fellow blond in New York as "one of the worst moments".
And he previously ridiculed the tycoon by saying "the only reason I wouldn't go to some parts of New York is the real risk of meeting Donald Trump".
Mr Johnson, who will become PM on Wednesday, has since changed his tack on the president.
As Foreign Secretary in 2016, he told Europeans to cease their "collective whinge-o-rama" over the tycoon's triumph.
And in the leadership campaign, Mr Johnson praised the president as having "many, many good qualities" and celebrated him for having "got the US economy motoring along".
He was also very cautious not to stray too far from the president's thinking, or criticise him too severely.
Mr Johnson was accused of having thrown ambassador to the US Sir Kim Darroch "under the bus" when he refused to say whether he would keep him in his post amid intense criticism from the president.
The following day Sir Kim resigned, outraging the diplomatic world.
And, while criticising Mr Trump's "go back" remarks to four congresswomen as completely unacceptable, he refused to call them racist, as many others did.
But Mr Johnson did demonstrate his willingness to differ from the president when it came to issues such as Iran, saying in the campaign that he supports diplomatic efforts for the time being.
Asked if he would back the US on an intervention, he said he does not believe war is a "sensible option".
Mr Johnson will hope he can maintain the close ties he currently has achieved with the American leader.
But, as Mrs May learned, the president's praise can turn into insults with the click of a button.
A month after his state visit, Mr Trump tweeted criticism of her Brexit negotiations and accused her of having gone "her own foolish way".