At a campaign rally in Montana, Trump gleefully recounted the attack on reporter Ben Jacobs of The Guardian, when Gianforte grabbed the journalist by the neck and threw him to the ground.
He was ordered to pay $385 (£304), complete 40 hours of community service and 20 hours of anger management counselling. Gianforte admitted Jacobs had done nothing wrong.
The attack came a day before Mr Gianforte won a 2017 special election to serve the remaining 18 months in the House term vacated by now-Interior Secretary Ryan Zinke. "I think it might help him, and it did", Trump said to applause from the 4,000 rally attendees.
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Trump, who has called the media the enemy of the American people and regularly derides journalists as "fake news", made his latest remarks during a campaign rally in Montana.
Trump also recounted how he initially heard about the incident, and why he believed it to be good news for Gianforte's campaign as voters headed to the polls.
"And then I said, well, wait a minute".
It also came amid global furor over the disappearance of journalist Jamal Khashoggi, last seen entering the Saudi consulate in Turkey earlier this month.
The Guardian US editor John Mulholland took aim at Trump's remarks. Turkish authorities say that he was murdered in the consulate, and his apparent grisly killing has turned attention to the US relationship to the regime of Saudi Crown Prince Mohammed bin Salman.