"I could not be prouder to congratulate our next President, Joe Biden, and our next First Lady, Jill Biden," the statement began.
Mr. Obama also praised Harris' "groundbreaking election as our next Vice President." Harris, elected as a senator from California in 2016, has now made history as the first woman and first person of color to be chosen to serve as vice president.
Mr. Obama continued: "We're fortunate that Joe's got what it takes to be President and already carries himself in that way."
He said Biden would "face a series of extraordinary challenges no incoming President ever has," including the coronavirus pandemic and a global shift toward authoritarian rule.
A spokesperson for Mr. Obama confirmed that the former president spoke separately with both Biden and Harris Saturday afternoon to offer his congratulations, "and told them how proud he was of the campaign they ran in unprecedented times."
CBS News projected on Saturday that Biden will win Pennsylvania, with 20 electoral votes, and Nevada, with six, giving him a decisive 279 electoral vote majority.
Former Presidents Bill Clinton and Jimmy Carter also congratulated the president-elect.
"America has spoken and democracy has won. Now we have a President-Elect and Vice President-Elect who will serve all of us and bring us all together. Congratulations to Joe Biden and Kamala Harris on your momentous victory!" Mr. Clinton wrote on Twitter.
Mr. Carter, who before Mr. Trump's projected defeat was the last incumbent president to lose a reelection bid, congratulated Biden and Harris via a statement posted to Twitter by the Carter Center. "We are proud of their well-run campaign and look forward to seeing the positive change they bring to our nation," he said.
It comes after former President Obama spent the final days of the campaign stumping for his former running mate, including speaking at drive-in rallies in key battleground states that were essential to Biden's victory.
Mr. Obama also thanked those who volunteered for the Biden campaign and all Americans who participated in the electoral process, including "everybody who voted for the first time."
He encouraged everyone who supported Biden's campaign to "enjoy this moment," but urged them to "stay engaged" in the democratic process.
"I know it can be exhausting," he wrote. "But for this democracy to endure, it requires our active citizenship and sustained focus on the issues — not just in an election season, but all the days in between."
The 2020 election cycle saw record voter participation, with some states shattering their early voting and total voter turnout records. While the final votes are still being tallied, Biden already made history for being elected with theever cast for a presidential candidate.
Mr. Obama concluded his statement with a promise to the new president-elect: "Michelle and I look forward to supporting our next President and First Lady however we can."