“I think he is going to be a great prime minister,” Trump said at the White House.
“I think he will be far superior,” he added, a possible reference to former premier Theresa May, to whom Trump often offered unsolicited advice on how to handle Brexit negotiations.
The U.S. administration says it wants to reach a trade deal with Britain as soon as the country exits the European Union.
“Dealing with the European Union is very difficult.
“They drive a hard bargain,” Trump said during a meeting with Romanian President Klaus Iohannis.
Johnson, seen as a Brexit hardliner, is due to visit Berlin on Wednesday ahead of the G7, but Chancellor Angela Merkel has ruled out reopening talks on leaving the EU.
Johnson has insisted that EU leaders scrap the so-called backstop, aimed at avoiding a hard border between Northern Ireland and the Republic of Ireland.