Majette, a first-term congresswoman from the Atlanta suburb of Stone Mountain, defeated Cliff Oxford by 20 percentage points, the Atlanta Journal-Constitution reported Wednesday.
Majette will face veteran Rep. Johnny Isakson, R-Ga., in the general election Nov. 2, and is considered a long shot.
Isakson has a well-financed campaign and has momentum from a victory without a runoff in a three-way Republican contest earlier this summer.
If Isakson wins, it will be the first time both of Georgia's senators are Republicans. Sen. Saxby Chambliss was elected two years ago.
At stake this year is the seat vacated by retiring Sen. Zell Miller, a Democrat who frequently voted with the GOP.
In another runoff Tuesday, Republican state Sen. Tom Price won a seat in the House by defeating state Sen. Robert Lamutt. Price does not have a Democratic opponent in November.