Joe Biden’s victory comes after four days of uncertainty as election officials sorted through a surge of mail-in votes that delayed the processing of some ballots. Biden secured 270 electoral college votes with a win in Pennsylvania.
Major news networks have called the battleground state of Pennsylvania for Joe Biden, projecting him 46th "president-elect".
The victory was sealed after the Democratic nominee claimed several key battleground states won by the Republican incumbent in 2016.
CNN, Reuters, New York Times and CBS News called the race in his favour.
Biden's running mate Kamala Harris will make history by becoming the first vice president-elect who is a woman, a Black and an Indian American.
Biden, 77, will be the oldest candidate ever elected to the White House. Biden spent eight years as vice president to Barack Obama.
His victory comes in his third run for the nation's highest office.
Biden said on Saturday he was honoured that Americans have chosen him to be their president and said it was now time to heal divisions left by the election campaign and unite as a country.
"I am honored and humbled by the trust the American people have placed in me and in Vice President-elect Harris. In the face of unprecedented obstacles, a record number of Americans voted," Biden said on Twitter.
"With the campaign over, it's time to put the anger and the harsh rhetoric behind us and come together as a nation. It’s time for America to unite.
America, I’m honored that you have chosen me to lead our great country.— Joe Biden (@JoeBiden) November 7, 2020
The work ahead of us will be hard, but I promise you this: I will be a President for all Americans — whether you voted for me or not.
I will keep the faith that you have placed in me. pic.twitter.com/moA9qhmjn8
President Donald Trump, who has made repeated claims of electoral fraud without providing proof, immediately accused Biden of "rushing to falsely pose as the winner."
"This election is far from over," he said in a statement.
State elections officials across the country say there has been no evidence of fraud.
Biden's projected victory came after four days of nail-biting suspense over the outcome of Tuesday's election, with the counting of votes in a handful of battleground states still going on thanks to a flood of mail-in ballots.
Elbow bumps and banging pots
As the news broke, loud cheers erupted in the halls of the hotel where Biden aides were staying and around the country.
"Worth every minute," of the wait, a Biden aide said, as campaign staff exchanged elbow bumps and air hugs in the lobby.
Cheers and applause was heard in neighborhoods in Washington, DC In one community, people emerged onto balconies, yelling, waving and banging pots. The wave of noise built as more people learned of the news.
Some were in tears. Music began to play, "we are the champions" blared.
In the Brooklyn neighborhood of Bedford-Stuyvesant, people clapped, honked car horns and erupted in screams of joy as the news spread of Biden's victory. Some residents danced on a building's fire escape, cheering while others screamed "yes!" as they passed by.
The networks' declaration that Biden had won came amid internal concerns within Trump's team about the strategy going forward and pressure on him to pick a more professional legal team to outline where they believe voter fraud took place and show evidence pointing toward it.
One Trump loyalist said Trump simply was not ready to admit defeat even though there would not be enough ballots thrown out in a recount to change the outcome. "There's a mathematical certainty that he's gong to lose," the loyalist said.
When Biden enters the White House on January 20, the oldest person to assume the office at age 78, he likely will face a difficult task governing in a deeply polarised Washington, underscored by a record nationwide voter turnout in a fight to the finish.
The former vice president had a 273 to 214 lead in the state-by-state Electoral College vote that determines the winner, having won Pennsylvania's 20 electoral votes to put him over the 270 he needed to secure the presidency, according to Edison Research, New York Times, Reuters and other news organisations.
Most crucial election since 1930s
To secure the win, Biden faced unprecedented challenges. These included Republican-led efforts to limit mail-in voting at a time when a record number of people were due to vote by mail because of the pandemic, which has killed more than 235,000 people in the United States.
Both sides characterized the 2020 election as one of the most crucial in US history, as important as votes during the 1860s Civil War and the 1930s Great Depression.
For months, officials on both sides raised the spectre of the United States not being able to pull off a fair vote. In the end, however, voting at the polls proceeded with limited disruption as millions lined up patiently to vote. Thousands of election monitors from both parties worked for four days to ensure the votes were being counted.
Biden's victory was driven by strong support from groups including women, African Americans, white voters with college degrees and city-dwellers.
He was more than four million votes ahead of Trump in the nationwide popular vote count.
Biden, who has spent half a century in public life as a U.S. senator and then vice president under Trump's predecessor Barack Obama, will inherit a nation in turmoil over the coronavirus pandemic and the related economic slowdown as well as disruptive protests against racism and police brutality.
Biden has said his first priority will be developing a plan to contain and recover from the pandemic, promising to improve access to testing and, unlike Trump, to heed the advice of leading public health officials and scientists.
Biden also has pledged to restore a sense of normalcy to the White House after a presidency in which Trump praised authoritarian foreign leaders, disdained longstanding global alliances, refused to disavow white supremacists and cast doubt on the legitimacy of the US election system.
Despite his victory, Biden will have failed to deliver the sweeping repudiation to Trump that Democrats had hoped for, reflecting the deep support the president enjoys despite his tumultuous four years in office.
This could complicate Biden's campaign promises to reverse key parts of Trump's legacy.
These include deep Trump tax cuts that especially benefited corporations and the wealthy, hardline immigration policies, efforts to dismantle the 2010 Obamacare healthcare law and Trump's abandonment of such international agreements as the Paris climate accord and Iran nuclear deal.
Should Republicans keep control of the US Senate, they would likely block large parts of his legislative agenda, including expanding healthcare and fighting climate change. That prospect could depend on the outcome of four undecided Senate races, including two in Georgia.
Biden, set to become the 46th US president, mounted unsuccessful bids for the presidency in 1988 and 2008. Harris, his running mate, will become the first woman, the first Black American and the first American of Asian descent to serve as vice president, the country's No 2 office.