The contest was the most expensive House campaign in US history
The Republican candidate Karen Handel has won a special election in Georgia, narrowly ahead of her Democratic rival Jon Ossoff.
It was the second round of voting, with a runoff election triggered after Mr Ossoff ended up less than 2% short of a majority in the initial vote in April.
Final results for the runoff showed Mrs Handel on 51.9% of the vote, with Mr Ossoff on 48.1%.
The New York Times reports that the contest was "the most expensive House campaign in history", with the young Democrat managing to raise $25 million from supporters hoping to turn the tide after last year's Republican victories in the presidential and congressional elections.
While a blow for Democrats, the vote marked a significant improvement for the party's showing in the district compared to 2016 - when Republican Tom Price (who resigned to become US Secretary of Health) secured 61.68% of the vote.
In a series of tweets after the vote, Mr Ossoff wrote: "For months, this community in Georgia has been a beacon of hope for people across the country. We showed the world that in places no one even thought it was possible to fight, we can fight. And we will fight.
"It's extraordinary what this community has achieved - an unprecedented grassroots effort. Thank you from the bottom of my heart."
Mrs Handel, meanwhile, was congratulated by President Trump, who had publicly commented on the race several times in recent days.
Congratulations to Karen Handel on her big win in Georgia 6th. Fantastic job, we are all very proud of you!— Donald J. Trump (@realDonaldTrump) June 21, 2017
Mr Trump also praised Ralph Norman - the Republican candidate in South Carolina - after his win in another special election yesterday.
Ralph Norman ran a fantastic race to win in the Great State of South Carolina's 5th District. We are all honored by your success tonight!— Donald J. Trump (@realDonaldTrump) June 21, 2017
Special elections were triggered in Kansas, Montana, Georgia and South Carolina after the sitting Congressmen resigned to take up senior roles in Washington. All those seats have now been retained by the Republicans.
Several of the races proved unexpectedly tight, amid efforts by Democrats and progressives to push back against the Trump administration and their policies.
The Montana election proved particularly dramatic, after the ultimately successful Republican candidate Greg Gianforte was charged with 'body slamming' a Guardian reporter on the eve of the election.
An additional election took place in California earlier this month, after Xavier Becerra resigned to become the state's attorney general. Jimmy Gomez took the seat in a vote against fellow Democrat Robert Lee Ahn.
A further special election will take place in Utah in November, following the resignation of Republican Jason Chaffetz.
Both major US parties will now be focusing on next year's midterm elections, when the Democrats will be hoping to make big gains in both houses of Congress.