A race that was expected to be a test of President Donald Trump's political influence ahead of next year's U.S. congressional elections was jolted by the charge against Gianforte, a wealthy technology executive who had urged voters to send him to Congress to help Trump.
Speaking to cheering supporters in Bozeman after his win, Gianforte apologized for the incident and said he was not proud of his actions.
"I should not have responded the way I did, and for that I'm sorry," Gianforte said. "I should not have treated that reporter that way."
Gianforte beat Democrat Rob Quist, a banjo player and first-time candidate who had focused his campaign on criticism of the Republican effort to repeal and replace former President Barack Obama's healthcare law. CNN projected Gianforte would win. With 96 percent of the vote counted, he led Quist by 51 percent to 43 percent.
Gianforte prevailed despite being charged on Wednesday night with misdemeanor assault on Ben Jacobs, a political correspondent for the U.S. edition of the Guardian newspaper, who said the candidate "body-slammed" him during a campaign event in Bozeman.
Gianforte's victory is a boost for Republicans, who are worried Trump's political stumbles and the unpopularity of the healthcare bill passed by the House will hurt their chances of holding on to a 24-seat House majority in next year's elections.
But the relatively close margin of the race in Republican-leaning Montana was encouraging to Democrats, who are already focused on next month's hotly contested special House election in the suburbs of Atlanta, Georgia.
Gianforte had been favored to win in Montana, where Republicans have held the lone House seat for two decades and where Trump won by more than 20 percentage points in the 2016 presidential election.
The race had grown closer in the last week, however, as Quist focused on criticism of the House healthcare bill.
Quist, wearing his signature cowboy hat, told supporters in Missoula, Montana, that the grassroots energy of his campaign would continue.
"I know that Montanans will hold Mr Gianforte accountable," Quist said.
It was unclear if Gianforte's assault had an impact on the vote. More than a third of the state's registered voters had already submitted ballots before it happened, state election officials said, and some Gianforte supporters shrugged off the charges or said they did not believe published accounts.
"I feel like, it's all just propaganda, you know what I mean, it's hard for me to believe anything the media tells me," said Nathaniel Trumper, who cast a vote for Gianforte at a polling station in Helena.
The assault occurred as Jacobs tried to ask Gianforte about healthcare, according to an audio tape. Fox News Channel reporter Alicia Acuna, who was preparing to interview Gianforte, said the candidate "grabbed Jacobs by the neck with both hands and slammed him to the ground."
Afterward, three state newspapers rescinded their endorsements of Gianforte. Some Republican lawmakers, including House Speaker Paul Ryan, suggested he apologize.
Gianforte specifically addressed his apology to Jacobs. "Last night I made a mistake," he said, adding: "I'm sorry, Mr Ben Jacobs."
Republican National Committee Chairwoman Ronna McDaniel called Gianforte's apology "a good first step toward redemption" and said she hoped he "continues to work toward righting his wrong."
Gianforte will take the House seat vacated when Trump named Ryan Zinke as secretary of the interior.
Trump and Vice President Mike Pence recorded robocalls to voters on Gianforte's behalf, and Republican groups poured millions into ads criticizing Quist for property tax liens and unpaid debts, which Quist said stemmed from a botched gallbladder surgery.
Quist, who raised more than $6 million for his upstart bid, said the experience gave him insight into the economic struggles some people face. He campaigned last weekend with U.S. Senator Bernie Sanders of Vermont, who won the state's 2016 Democratic presidential primary against Hillary Clinton.
Gianforte could face additional, more serious charges once prosecutors review the evidence, Gallatin County Attorney Marty Lambert told Reuters.
Gianforte has two weeks to enter a plea to the misdemeanor citation issued by the Gallatin County Sheriff's Office, according to Lambert, who said he would likely review the case before then to decide whether it should be treated as a felony offense, which would supersede the current charge.
"There's always the possibility that when we get the case and the details, that we might look differently at the charging decision," Lambert said.