The Republican technology entrepreneur will enter his plea in court on Monday, when he is scheduled to be arraigned and sentenced on the misdemeanor charge, Gallatin County Attorney Marty Lambert told The Associated Press.
Gianforte requested the court hearing after reaching a civil settlement this week.
Guardian reporter Ben Jacobs said Gianforte knocked Jacobs to the ground when the reporter asked him a question May 24.
As part of the settlement, Jacobs said he would not object to Gianforte entering a plea of no contest, or nolo contendere, meaning Gianforte would concede to the charge without admitting guilt.
But Lambert said Gianforte will plead guilty.
"He is not going to be entering a nolo contendere plea," Lambert said. "He's going to be pleading guilty."
Gianforte spokesman Shane Scanlon did not return telephone and text messages seeking comment.
Misdemeanor assault convictions in Montana carry a maximum penalty of six months in prison and a $500 fine.
Lambert said he will recommend a penalty to Justice of the Peace Rick West at Monday's hearing, but declined to disclose his recommendation.
Jacobs said Gianforte "body slammed" him after the reporter asked a question about the Congressional Budget Office's analysis of a health care bill the day before the May 25 election.
Audio of the encounter taken by Jacobs recorded sounds of a scuffle, followed by Gianforte yelling for the reporter to "get the hell out of here." Police charged Gianforte that night.
Jacobs signed a release agreeing not to sue Gianforte. In exchange, Gianforte wrote a letter of apology to Jacobs and pledged to donate $50,000 to the Committee to Protect Journalists.
Both Gianforte and Jacobs have said in statements they want to put the episode behind them.
Gianforte is expected to be sworn in next month after defeating Democrat Rob Quist in the special congressional election to serve the remainder of Interior Secretary Ryan Zinke's term.
Zinke won a second term as Montana's congressman last November before President Donald Trump tapped him for the cabinet post.
That set off a short and intense special election campaign that drew millions of dollars in donations from across the U.S.
Republicans vigorously campaigned to defend the seat they had held for decades. Democrats sought to capitalize on a wave of activism triggered by Trump's election in an attempt to take it back for the first time since 1997.
The 56-year-old Gianforte, coming off an unsuccessful bid for governor last fall, defeated Quist, the front man for the popular musical group Mission Mountain Wood Band, by six percentage points.
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