Republican incumbent Sen. David Perdue holds just under 50% of the vote in the state, closely trailed by Democratic challenger Jon Ossoff.
Elections have been particularly close in Georgia, with Secretary of State Brad Raffensperger saying Friday that the contest between President Trump and Democratic nominee Joe Biden was heading toward a statewide recount.
Georgia election law requires a candidate to secure over 50% of the vote. If no candidate is able to do so, then the top two candidates in the race face off in a runoff election.
Ossoff predicted the call at a press conference Friday morning, standing in front of a small crowd.
"We have all the momentum, we have all the energy, we're on the right side of history. Y'all ready to work? We're just getting started," he said.
The runoff will be held Jan. 5 and set the stage for what could be an especially contentious and expensive battle for control of the Senate. A special election between Republican Sen. Kelly Loeffler and Democrat Raphael Warnock for the state's other Senate seat is already headed for a runoff that day.