"There is no one better prepared than Joe to steer our nation through these turbulent times, and restore truth, honor, and decency to the Oval Office. He is kind and endlessly caring, and he truly listens to the American people," Harris, the Democratic senator from California, said in a statement.
"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."
At one time, more two dozen Democrats were seeking the presidency.
But after Biden's decisive victory over Sanders in South Carolina's primary eight days ago, several candidates dropped out: billionaire Tom Steyer, former South Bend Mayor Pete Buttigieg, Sen. Amy Klobuchar of Minnesota, billionaire and former New York City Mayor Mike Bloomberg, and Sen. Elizabeth Warren of Massachusetts.
All but Steyer and Warren have endorsed Biden, their one-time rival.
Former U.S. Rep Beto O'Rourke, also a former presidential candidate, endorsed Biden in a rally in Dallas the night before Super Tuesday with Klobuchar. Biden went on to win 10 of 14 states and has a 625-548 lead, according to the CBS delegate tracker. A total of 1,991 votes are needed to win the Democratic nomination in Milwaukee, Wis., this summer. Some of the candidates who dropped out earned delegates.
Gabbert has two delegates.
Harris noted the withdrawal of several women.
"This is something we must reckon with and it is something I will have more to say about in the future," Harris wrote. "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."
Harris, who was elected to the Senate in 2016, also spoke of her endorsement of Biden endorsement in a video, saying "One of the things we need right now is a leader who really does care about the people and who can therefore unify the people, and I believe Joe can do that."
During Democratic president debates, Harris confronted Biden on his opposition to desegregation busing.
But after she dropped out of the race, Biden said he would consider the only African-American female candidate as a potential running mate.
"She is solid. She can be president someday herself. She can be the vice president. She can go on to be a Supreme Court justice. She can be an attorney general. She has enormous capability," Biden said.
Six states will conduct primaries Tuesday: Idaho, Michigan, Mississippi, Missouri, North Dakota and Washington.