The US vice-president has already secured the support of a majority of Democratic delegates, setting her on course to become the official nominee at the party convention in August.
The Obamas said in Friday's statement that they could not be "more thrilled to endorse" Ms Harris. They vowed to do "everything we can" to elect her.
"We agree with President Biden," said the couple's statement, "choosing Kamala was one of the best decisions he’s made. She has the resume to prove it."
They cited her record as California’s attorney general, a US senator and then vice-president.
"But Kamala has more than a resume," the statement continued. "She has the vision, the character, and the strength that this critical moment demands.
"There is no doubt in our mind that Kamala Harris has exactly what it takes to win this election and deliver for the American people.
"At a time when the stakes have never been higher, she gives us all reason to hope."
The statement was accompanied by a video of Ms Harris taking a phone call from the Obamas in which they pledge their support.
"Oh my goodness," says the vice-president in the clip. "Michelle, Barack, this means so much to me."
Ms Harris has held a number of campaign events across the US in the days since Mr Biden withdrew from the race.
On Thursday, she addressed the American Federation of Teachers union in Houston.
While the event was billed as part of her vice-presidential duties, it had the air of a campaign rally, with loud applause and cheers from the audience.
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