The technology entrepreneur on Tuesday successfully defended the seat he won in a special election last year by campaigning as a strident supporter of President Donald Trump.
He called the race a choice between standing with Trump or going back to the era of former President Barack Obama.
Gianforte is best known nationally for pleading guilty to assaulting Guardian reporter Ben Jacobs in 2017. The attack emerged as a campaign issue when Trump praised Gianforte for it during an October rally.
Williams is a former state lawmaker who benefited from a fundraising surge in the final months of the race. She sought to become the first Democrat in Montana's only House seat in 22 years.