Democratic presidential candidate Joe Biden has named Kamala Harris as his running mate - the first black woman and South Asian American in the role.
Once a rival for the top job, the California senator of Indian-Jamaican heritage had long been considered the front-runner for the number two slot.
The former California attorney general has been urging police reform amid nationwide anti-racism protests.
Mr Biden will face President Donald Trump in the election on 3 November.
After Tuesday's announcement, Ms Harris tweeted that Mr Biden "can unify the American people because he's spent his life fighting for us. And as president, he'll build an America that lives up to our ideals.
"I'm honored to join him as our party's nominee for Vice President, and do what it takes to make him our Commander-in-Chief."
Mr Biden and Ms Harris will deliver remarks in Wilmington, Delaware, on Wednesday afternoon on "working together to restore the soul of the nation and fight for working families to move the country forward", the Biden campaign said.
At a White House news conference on Tuesday, Mr Trump, a Republican, said he was pleased with Mr Biden's choice, adding that Ms Harris did "very, very poorly" in her effort to become the Democratic nominee.
Ms Harris will take part in a debate with Mr Trump's running mate, Vice-President Mike Pence, on 7 October in Salt Lake City, Utah.
In last year's race to be the Democratic nominee, Kamala Harris showed herself to be a forceful speaker, launching blistering attacks on Donald Trump.
The role of a vice-presidential running mate isn't always clearly defined.
One of the traditional roles is to go on the offensive in exposing the opposition's weaknesses, while the presidential nominee focuses on communicating the party's message, says the BBC's North America reporter Anthony Zurcher.
Constitutionally, the vice-president steps in to the top job should the president die or leave office during his or her term.
Mr Biden will turn 78 in November, meaning should he be elected he will be the oldest US president in history (Ronald Reagan was 77 when he left office).
His age means Mr Biden's vice-presidential choice may come under extra scrutiny.
Only two other women have been nominated as vice-presidential candidates for a major party - Sarah Palin by the Republican party in 2008 and Geraldine Ferraro by the Democrats in 1984. Neither were on the winning ticket.
A woman of colour has never been appointed to a presidential ticket by either of the two main American political parties. No woman has won the US presidency either.
What did Biden say?
Mr Biden tweeted that he had "the great honour" to name Ms Harris as his number two.
He described her as "a fearless fighter for the little guy, and one of the country's finest public servants".
He noted how she had worked closely with his late son, Beau, when she was California's attorney general.
"I watched as they took on the big banks, lifted up working people, and protected women and kids from abuse," he tweeted.
"I was proud then, and I'm proud now to have her as my partner in this campaign."
Mr Biden pledged in March to name a woman on the ticket. He had faced mounting calls to pick a black woman in recent months as the nation has been convulsed by social unrest over police brutality against African Americans, a key voting bloc for the Democratic Party.
Who is Kamala Harris?
Ms Harris, 55, dropped out of the presidential race in December after failing to make headway in her bid to win the Democratic nomination.
She repeatedly clashed with Mr Biden during the primary election debates, most notably criticising his praise for the "civil" working relationship he had with former senators who favoured racial segregation.
Ms Harris was born in Oakland, California, to two immigrant parents: an Indian-born mother and Jamaican-born father.
She went on to attend Howard University, one of the nation's preeminent historically black colleges and universities. She has described her time there as among the most formative experiences of her life.
Ms Harris says she's always been comfortable with her identity and simply describes herself as "an American".
In 2019, she told the Washington Post that politicians should not have to fit into compartments because of their colour or background. "My point was: I am who I am. I'm good with it. You might need to figure it out, but I'm fine with it," she said.
The obvious pick
Sometimes the obvious pick is obvious for a reason. Kamala Harris was the front-runner to be Joe Biden's running mate pretty much since the moment the presumptive Democratic nominee announced in March that he would pick a woman to be his ticket.
She's relatively young and telegenic, and as the daughter of Jamaican and Indian immigrants reflects the growing diversity of the Democratic Party.
What's more, she's been in the spotlight of the national media, having campaigned for president in 2019 and, for a while last summer, risen to near the top of some polls. Many of her rivals for the number-two spot had never faced such scrutiny, so there was no proof that they could hold up under fire.
Another underrated advantage for Ms Harris was her friendship with Mr Biden's late son, Beau, formed when they were both attorneys general. Mr Biden places a high value on family - and that connection may have made choosing her easier.
Now Ms Harris will have a chance to hit the campaign trail again and prove that she deserves this historic pick. If she succeeds, she'll be in prime position to seek the presidency again, perhaps as early as 2024. Today has made her a force in the Democratic Party for years to come.
What is her record?
After four years at Howard, Ms Harris went on to earn her law degree at the University of California, Hastings, and began her career in the Alameda County District Attorney's Office.
She became the district attorney - the top prosecutor - for San Francisco in 2003, before being elected the first woman and the first African American to serve as California's attorney general, the top lawyer and law enforcement official in America's most populous state.
In her nearly two terms in office as attorney general, Ms Harris gained a reputation as one of the Democratic party's rising stars, using this momentum to propel her to election as California's junior US senator in 2017. She was only the second black woman ever elected to the US senate.
She launched her candidacy for president to a crowd of more than 20,000 in Oakland at the beginning of last year.
But Ms Harris failed to articulate a clear rationale for her campaign, and gave muddled answers to questions in key policy areas like healthcare.
She was also unable to capitalise on the clear high point of her candidacy: debate performances that showed off her prosecutorial skills, often placing Mr Biden in the line of attack.
The self-described "progressive prosecutor" tried to emphasise more left-leaning parts of her legacy - requiring body cameras for some special agents at the California Department of Justice, the first state agency to adopt them, and launching a database that provided public access to crime statistics, though she failed to gain traction.
"Kamala is a cop" became a common refrain on the campaign trail, spoiling her attempts to win over the more liberal Democratic base during the primaries. Those same law enforcement credentials could, however, prove beneficial in the general election when Democrats need to win over more moderate voters and independents.
What's the reaction?
President Trump told reporters: "She's a person that's told many, many stories that weren't true."
He added Ms Harris was "very, very nasty" and "horrible" to Mr Biden during the Democratic primary debates.
"She was very disrespectful to Joe Biden and it's hard to pick somebody that's that disrespectful," he said.
The Trump campaign said the choice of running mate was proof that Mr Biden is "an empty shell being filled with the extreme agenda of the radicals on the left".
The unsuccessful Democratic candidate in 2016, Hillary Clinton, said she was "thrilled to welcome [Ms Harris] to a historic Democratic ticket".
"She's already proven herself to be an incredible public servant and leader," she added.
Former US President Barack Obama - whom Mr Biden served as vice-president for eight years - tweeted: "She is more than prepared for the job. She's spent her career defending our Constitution and fighting for folks who need a fair shake.
"This is a good day for our country. Now let's go win this thing."