"There is no one better prepared than Joe to steer our nation through these turbulent times, and restore truth, honour, and decency to the Oval Office," Harris said in a statement.
"I believe in Joe Biden and will do everything in my power to help elect him the next president of the United States," she said.
The California Senator had dropped out of the Democratic presidential race even before the start of the primaries.
She had made an impressive performance against Biden during the first Democratic party presidential debate last summer. However, she could not keep up the momentum and her poll numbers started dropping.
Biden is kind and endlessly caring, and truly listens to the American people, Harris said.
"You can see in his eyes how he takes to heart the experiences of mothers and fathers working to make ends meet and worrying about whether their children can be safe in their classroom, or young people who fight tirelessly to tackle climate change as they ask for a fair shot at the future in front of them. And with a lifetime in public service, Joe has a proven track record of getting things done," she said.
Widely being speculated as a potential vice presidential candidate, Harris said that the US was at an inflection point and the decision voters make this November will shape the country and the world "our children and grandchildren will grow up in".
Biden has said he would like to have a woman as his running mate. However, he has not given any indication who that woman would be.
"Like many women, I watched with sadness as women exited the race one by one. Four years after our nominee, the first woman to win the nomination of a major party, received 3 million more votes than Donald Trump but still lost, we find ourselves without any woman on a path to be the Democratic nominee for president," Harris said.
"This is something we must reckon with and it is something I will have more to say about in the future. But we must rise to unite the party and country behind a candidate who reflects the decency and dignity of the American people and who can ultimately defeat Donald Trump," she said.
Harris' endorsement comes as the Democratic field has been winnowed down to just two major candidates - Biden and Vermont Senator Bernie Sanders.
Biden, after his Super Tuesday turnaround, holds a narrow advantage over Sanders in the chase for Democratic delegates who will decide the nomination at the party's convention in Milwaukee in July.
Barack Obama's vice-president for eight years has so far secured 664 of the 1,991 state delegates he needs, while Sanders has 573.
On Tuesday, a further 352 delegates are at stake in primary votes in Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington.
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