"With the margin being so close, it will require a full, by-hand recount in each county," Raffensperger said at a press conference.
Raffensperger, a Republican, said the state will work with county officials to complete the recount in time to meet its Nov. 20 deadline for certifying statewide election results.
"It'll take every bit of the time we have left, for sure," he said.
Biden's lead over Trump in Georgia has grown to more than 14,000 votes among nearly 5 million cast, according to NBC News, which has not projected a winner in the Peach State.
Raffensperger had previously telegraphed that a recount was likely in the state, which has traditionally voted for Republican presidential nominees. Trump's supporters, including GOP Rep. Doug Collins, asked the Georgia official earlier this week for a hand recount.
"This will help build confidence. It will be an audit, a recount and a recanvass all at once," Raffensperger said. "It will be a heavy lift but we will work with the counties to get this done in time for our state certification."
Trump, on the day after the Nov. 3 election, falsely asserted that his campaign had "claimed, for Electoral Vote purposes," Georgia and other states. With Biden now projected to secure more than the 270 electoral votes required to clinch the presidency, Trump and his surrogates have already demanded recounts in states including Georgia and Wisconsin.
They have also launched a flurry of accusations of electoral fraud, without citing evidence. Trump campaign lawyers have filed lawsuits in multiple key states related to those claims. Many of those cases have already been thrown out by judges.
Trump has refused to concede to Biden and is falsely claiming he won the election. The transition of power to a Biden administration cannot formally begin until the General Services Administration makes an "ascertainment" of the winner, which it has so far refused to do.
Raffensperger said at the press conference that at 1 p.m. ET, he will declare the presidential race in his state is subject to a "risk-limiting audit."
"We'll be counting every single piece of paper, every single ballot. every single lawfully cast, legal ballot," he said.
When a reporter noted that such an audit usually involves just a sample of the ballots, rather than the entire body of votes, Raffensperger said, "you actually have to do a full hand-by-hand recount of all [ballots] because the margin is so close."
Raffensperger came under attack from within his own party on Monday, when Georgia's two Republican senators, Kelly Loeffler and David Perdue, demanded his resignation. The senators claimed in a joint statement that there have been "too many failures in Georgia elections this year," without citing any specific examples.
Both senators are closely allied with Trump likely facing runoff elections in January against their respective Democratic challengers.