Biden posted on X that he would speak "on what lies ahead" and how he will "finish the job for the American people".
He will speak at 8pm on Wednesday (10am AEST on Thursday).
The president departed Delaware shortly before 2pm on Tuesday, after nearly a week of isolating at his Rehoboth Beach home after his second bout with COVID-19.
Biden tested negative for the virus and his symptoms have resolved, according to a letter from his doctor Kevin O'Connor released on Tuesday.
Holding a blue paper mask, he told reporters that "I am feeling well" but did not answer other questions, such as whether Harris can defeat Republican presidential nominee Donald Trump.
Tomorrow evening at 8 PM ET, I will address the nation from the Oval Office on what lies ahead, and how I will finish the job for the American people.— President Biden (@POTUS) July 23, 2024
Biden has not been seen publicly since July 17 but he called into a campaign meeting on Monday to address staff and express his support for Harris' bid to replace him a day after announcing he would leave the race.
Harris swiftly consolidated her party's support after Biden, 81, abandoned his re-election campaign under pressure from members of his party who worried about his ability to beat Trump or to serve for another four-year term.
She wrapped up the nomination on Monday night by winning pledges from a majority of the delegates who at next month's party convention will determine the nominee, the campaign said.
US President Joe Biden has been seen in public for the first time in almost a week. (EPA PHOTO)
Most Democratic lawmakers have lined up behind her candidacy, including the party's leaders in the Senate and House, Chuck Schumer and Hakeem Jeffries, who endorsed Harris on Tuesday at a joint press conference.
An unofficial survey of delegates by the Associated Press showed Harris with more than 2500 delegates, well over the 1976 needed for the nomination.
Delegates could still change their minds, but no one else received any votes in the AP survey; 54 delegates said they were undecided.
"Thank you, thank you, thank you everyone. Good afternoon Wisconsin. It is good to be back," Harris said on Tuesday at a rally in West Allis outside of Milwaukee - her first words from the campaign trail, in a state decided by only about 10,000 votes in 2020.
To chants of "Kamala, Kamala, Kamala!" Harris thanked statewide elected Democrats, governor Tony Evers and Tammy Baldwin.
"I had the privilege of serving with Tammy when I was in the United States Senate. And I know that the folks that are here are going to make sure you return her to Washington DC in November," she said.
"The path to the White House goes through Wisconsin," she said.
Harris led Trump 44 per cent to 42 per cent among registered voters in a Reuters/Ipsos poll, conducted on Monday and Tuesday after Biden dropped out of the contest on Sunday and endorsed Harris as his successor.
Previous surveys taken before Biden's exit suggested Harris and Trump were tied at 44 per cent a week ago and Trump ahead of her by a percentage point at the beginning of the month.
In all three cases, the difference was within the poll's three-point margin of error.
Biden's dramatic exit followed Trump's narrow survival of a July 13 assassination attempt that raised questions about security failures in the US Secret Service.
The agency director, Kimberly Cheatle, resigned on Tuesday after numerous lawmakers called for her to step down.
Trump and his allies have tried to tether Harris to some of Biden's more unpopular policies, including his administration's handling of the surge of migrants at the southern border with Mexico.
"Kamala Harris' dismal record is one of complete failure and utter incompetence. Her policies are Biden's policies, and vice versa," Trump spokesman Steven Cheung said.
Wisconsin is among a trio of Rust Belt states, along with Michigan and Pennsylvania, that are critical for Democrats' chances of defeating Trump.
"There are independents and young people who did not like their choices, and Harris has a chance to win them," said Paul Kendrick, executive director of the Democratic group Rust Belt Rising.
Democratic National Committee chair Jaime Harrison told NBC the party had to move quickly to get the ticket on ballots in all 50 states, and that the vice presidential pick needed to be made by August 7.
"This process is going to be fair, transparent, open but it's going to be fast," Harrison said.
with AP
