Karen Handel retained the Atlanta seat for the Republicans with 53 per cent of the vote, against 47 for her Democratic rival Jon Ossoff, the BBC reported on Wednesday.
House Speaker Paul Ryan congratulated Handel on a "hard-earned" victory.
Meanwhile, in South Carolina, Republican Ralph Norman beat Democrat Archie Parnell in a solidly conservative area.
President Trump congratulated both candidates.
On the victory in suburban Atlanta, Trump tweeted: "Congratulations to Karen Handel on her big win in Georgia 6th. Fantastic job, we are all very proud of you!"
Georgia's sixth district is a traditionally safe Republican seat but Democrats had hoped to capitalise on the President's low approval ratings, the BBC reported.
Spending on candidates was put at $56 million, making it the costliest Congressional election in US history.
Democrats have already suffered narrow defeats in Kansas and Montana this year.
Addressing jubilant supporters, Handel thanked key Republican figures including Trump.
"I need to also thank Speaker Ryan and the House leadership and so many of the members across this country. And a special thanks to the President," she said.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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