Speaking outside the White House, the president was asked if he intended to visit the Conservative MP and Brexit Party leader Nigel Farage during his UK state visit next week.
"Nigel Farage is a friend of mine, Boris is a friend of mine, they're great people," he said.
"They're two very good guys, very interesting people," he continued, before remarking on Mr Farage's performance in the European elections.
"And I think they're big powers over there, I think they've done a good job."
Asked whether that meant he would lend his support to either of the two politicians, he suggested it may not be "his business" to do so.
"I like them. They're friends of mine but I haven't thought about supporting them.
"Maybe it's not my business to support people. But I have a lot of respect for both of those men."
Mr Trump will arrive in London for a state visit next week. It had been suggested that he could use the opportunity to endorse Mr Johnson, who is one of many Conservative MPs running to be leader of the party and become prime minister.
The last time Mr Trump came to the UK, in July 2018, he caused controversy by suggesting that Boris Johnson would make a "great prime minister". The comments, came just days after Mr Johnson resigned as foreign secretary over Brexit, caused great embarrassment to Theresa May.
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