Hillary Clinton named Virginia Senator Tim Kaine as her vice presidential running mate, adding a centrist former governor of a crucial battleground state to the Democratic ticket.
In a text message to supporters, the presumptive Democratic nominee said: "I'm thrilled to tell you this first: I've chosen Sen. Tim Kaine as my running mate."
On Twitter a few seconds later, Mrs Clinton described Mr Kaine as "a man who's devoted his life to fighting for others".
She called him "a relentless optimist who believes no problem is unsolvable if you put in the work to solve it".
The announcement moved Mrs Clinton into the political spotlight a day after newly crowned Republican nominee Donald Trump ended his convention with a fiery address accusing his general election opponent of "terrible, terrible crimes".
Mr Kaine, 58, had long been a favourite for Mrs Clinton's ticket. He is fluent in Spanish and active in the Senate on foreign relations and military affairs.
Mr Kaine tweeted: "I'm honored to be her running mate."
The two will make their first appearance together on Saturday at a rally in Miami.
Mrs Clinton's decision caps a highly secretive, months-long process to find a political partner. She called Mr Kaine by phone around 7:30pm local time on Friday to offer him the job, and he accepted, according to a campaign aide.
She then called President Barack Obama to inform him of the decision.
Mrs Clinton was also considering agriculture secretary Tom Vilsack, a long-time friend of the candidate and former President Bill Clinton.
But Mr Kaine's strong ties to Virginia, a crucial general election battleground, and his foreign policy experience gave him the edge, according to a campaign source.
Mr Trump, in a text to his own supporters, said Mr Obama, Mrs Clinton and Mr Kaine were "the ultimate insiders" and implored voters to not "let Obama have a 3rd term".
Mrs Clinton made no mention of her impending choice during a sombre meeting on Friday with community leaders and family members affected by the Pulse nightclub shooting in Orlando and a later campaign rally in Tampa.
Mr Kaine won election to the Senate in 2012 after serving as Mr Obama's first chairman of the Democratic National Committee.
Before entering politics he was a lawyer who specialised in civil rights and fair housing. He learned Spanish during a mission trip to Honduras while in law school.
His wife, Anne Holton, is the daughter of a former Virginia governor, a former state judge and, currently, the state's education secretary. The couple have three children.
Meanwhile Mr Trump met supporters at his convention hotel in Cleveland after the end of his four-day coronation as head of the Republican Party.
He can nominee spent considerable time stoking the fire of his bitter quarrel with Republican former rival Ted Cruz.
"Ted, stay home," Mr Trump said, dismissing any interest in an endorsement the Texas senator refuses to provide. "Relax. Enjoy yourself."
The tycoon boasted of his TV ratings, primary victories and other achievements, including winning over his wife, Melania, in a stream-of-consciousness delivery with his vice presidential nominee, Indiana Governor Mike Pence, standing quietly nearby.
"I don't do anything unless I win," Mr Trump insisted.
He promised to work "so hard" as the nominee and vowed his campaign was "not going to disappear," even though he has no plans for this weekend and no events on his schedule for next week.