Former President Obama and ex-First Lady Michelle Obama said in a joint statement that they believe Ms Harris has the "vision, the character, and the strength that this critical moment demands".
Mr Obama was reportedly among more than 100 prominent Democrats Ms Harris spoke to after President Joe Biden announced last Sunday he was dropping out of the race.
In a statement at the time, Mr Obama praised Mr Biden's exit, but stopped short of endorsing Ms Harris.
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."
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