The senator from California, who ran for the nomination before dropping out in December, made the announcement on Sunday morning.
"When I started my run for president, I said America needs a president who reflects the decency and dignity of the American people; a president who speaks the truth; and a president who fights for those whose voices are too often overlooked or ignored. I still believe that to this day. That is why I am proud to announce I am endorsing my friend, Vice President Joe Biden, for President of the United States," Ms Harris said in a statement.
"I truly believe our nation is at an inflection point. And the decision voters make this November will shape the country and the world our children and grandchildren grow up in," she added. "I believe in Joe Biden and will do everything in my power to help elect him the next president of the United States."
A video was also shared on her Twitter page, where the senator expressed great "enthusiasm" to be endorsing Mr Biden.
Mr Biden shared the senator's endorsement on his own Twitter page and thanked her.
"Kamala — You’ve spent your whole career fighting for folks who’ve been written off and left behind — and no small part of that alongside Beau. From our family: thank you," he wrote.
He mentioned his late son, Beau Biden, because Ms Harris served as California attorney general at the same time the son served as Delaware attorney general.
"I got to know Joe more than a decade ago through his son — my dear friend the late Delaware Attorney General Beau Biden," Ms Harris wrote in her statement. "As attorneys general during the financial crisis, Beau and I were leaders in the fight to take on the biggest banks in the nation and secure billions of dollars in relief for homeowners across the country. I can tell you that Beau inherited his strength of character, selfless courage, and commitment to public service from his father."
When Ms Harris was in the race for president, she clashed with the vice president about his past policy decisions. In the first debate, the Ms Harris, who is black, spoke about the issue of race in America and the "very hurtful" comments Mr Biden made when defending his work with two segregationist senators.
She also brought up Mr Biden's opposition to school busing in the 1970s.
"There was a little girl in California who was a part of the second class to integrate her public schools and she was bused to school every day," she said during the debate. "And that little girl was me."
Mr Biden defended himself by saying it was a "mischaracterisation" of what he stood for at the time, but the vice president was a leading opponent of school busing in the Senate in the 1970s and 1980s.
But now with the senator out of the race, she's put her differences with Mr Biden aside to support him in his bid for the Democratic nomination.
Her endorsement joins the other prominent members of the Democratic party who've put their support behind the vice president in recent days, including Senator Amy Klobuchar, former Mayor Pete Buttigieg, former Representative Beto O'Rourke, and Representative Jim Clyburn.
All of these endorsements have helped boost Mr Biden to lead the number of delegates over Bernie Sanders.
The next test comes on Tuesday when six states vote to choose a Democratic candidate: Michigan, Washington, Idaho, Mississippi, Missouri and North Dakota.
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