"We need a leader who really does care about the people, and can therefore unify the people," the senator added.
Mr Biden later thanked Ms Harris for her support. "Kamala - You've spent your whole career fighting for folks who've been written off and left behind… from our family: thank you," he tweeted.
The two have, however, previously clashed during the presidential debates.
Last June, Ms Harris attacked Mr Biden over his previous opposition to a policy combating segregation in schools.
Ms Harris - the only black woman in the Democratic field - pilloried Mr Biden for having recently reminisced about working with two Democratic senators who favoured racial segregation.
She said at the time that she did not believe he was a racist, but added: "It was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country."
Mr Biden's wife, Jill, said the senator's comments were "the biggest surprise" of the campaign, adding: "The one thing you cannot say about Joe is that he's a racist... I mean, he got into politics because of his commitment to civil rights."
Ms Harris, a fierce critic of Mr Trump, said she would be campaigning with Mr Biden in Detroit on Monday.
Separately, US civil rights activist Jesse Jackson on Sunday endorsed Mr Sanders, saying the Vermont senator's progressive social and economic policies would give black Americans "the best chance to catch up".
