Republicans Kelly Loeffler and David Perdue are neck and neck with Democrats Raphael Warnock and Jon Ossoff.
US President-elect Joe Biden’s Democrats need to win both seats to gain full control of Congress.
Ms Loeffler is taking on Reverend Warnock and Mr Perdue is battling former filmmaker Jon Ossoff.
All four candidates were in a dead heat with 75% of ballots counted.
None of the candidates reached the 50% needed to win outright in the elections in November, forcing Tuesday’s runoff elections under Georgia’s election rules.
What do the exit polls say?
So far, exit polls show Georgians in a clean split over which party they want to control Congress: 49% favoured Republicans, while 48% said the Democratic party.
The demographics roughly matched that of November’s election. Black voters made up 29% of the vote, and these voters favoured the Democratic candidates nine-to-one. The Republicans, meanwhile, were winning a majority of white voters.
Meanwhile, Mr Trump’s unproven claims of voter fraud may have eroded voter confidence in the election system’s integrity. According to exit polls from Edison Research, around 70% of voters were very or somewhat confident their votes would be counted accurately, a nearly 15% drop from the 3 November election.
On Saturday, Mr Trump pushed Georgia’s top election official Brad Raffensperger, a Republican, to “find” enough votes to overturn Mr Biden’s win in the state from November’s White House election.
Exit polls typically include interviews with voters after they have cast their ballot. These include people who voted early and on election day. Only a small number of voters are interviewed for exit polls so the results can differ from the official count.