His remarks come amid furore over fate of journalist Jamal Khashoggi who is presumed to be murdered.
Trump said he initially thought Gianforte's act of violence would hurt his election chances, but "then I said 'well, wait a minute, I know Montana pretty well, I think it might help him.' And it did!"
"Any guy that can do a body slam. he's my guy", he said to cheers and laughter at a rally in Montana.
He was ordered to pay $385 (£304), complete 40 hours of community service and 20 hours of anger management counselling.
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Permission apparently came after a late Sunday night call between King Salman and Turkish President Recep Tayyip Erdogan. Khashoggi was once a prominent member of the Saudi kingdom and a top adviser to the royal family.
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During his Thursday night Montana rally, President Donald Trump praised Republican Rep. Greg Gianforte for assaulting a reporter- even while we are in the midst of unanswered question about missing Washington Post journalist Jamal Khashoggi. "I shouldn't say that - (but) there's nothing to be embarrassed about".
"He's a great guy, tough cookie", he said. "I think it might help him, and it did'". He also donated $50,000 to the Committee to Protect Journalists.
US President Donald Trump on Thursday praised a US congressman who assaulted a reporter, making light of the attack.
On Mr Biden challenging him to a fight, Mr Trump said, "He'd be down, faster than Greg would take him down".
Trump is on a three-state tour in the western part of the United States advocating for Republican candidates who are running for election in November with the hope of keeping control of the US Senate and House of Representatives in Republican hands.