Washington: US President Donald Trump has praised Montana Republican Representative Greg Gianforte for assaulting a reporter during his campaign last May and called him "my guy".

"Any guy who can do a body slam ... he's my guy," Trump said at a Montana rally on Thursday night and made a gesture mimicking a body slam, according to CNN.

"I shouldn't say this," but "there's nothing to be embarrassed about," the president said amid laughter and applause from the crowd.

Gianforte pleaded guilty to misdemeanour assault in June 2017 after he was convicted of "body slamming" The Guardian reporter Ben Jacobs. A judge sentenced him to a 180-day deferred sentence, 40 hours of community service, 20 hours of anger management and a $300 fine along with a $85 court fee.

Trump said he found out about Gianforte assaulting a reporter when he was travelling in Rome and said initially he was concerned it would hurt the Republican in the election.

"Then I said, well wait a minute, I know Montana pretty well, I think it might help him. And it did." He called Gianforte, "one of the most respected people in Congress" and a "tough cookie".

Gianforte won the election the next day and apologized to Jacobs during his acceptance speech.

Guardian US editor John Mulholland slammed Trump's joke, saying: "To celebrate an attack on a journalist who was simply doing his job is an attack on the First Amendment by someone who has taken an oath to defend it."