The choice is unlikely to appeal to supporters of Clinton's former rival, democratic socialist Senator Bernie Sanders
Hillary Clinton announced on Friday that she has chosen Tim Kaine, a Catholic senator from the battleground state of Virginia, to be her White House running mate.
"I'm thrilled to announce my running mate, @timkaine, a man who's devoted his life to fighting for others. -H," the presumptive Democratic nominee tweeted.
In a second Twitter message, Clinton heaped praised on Kaine, 58, known for his affable manner and expertise on matters of defense and foreign policy.
"@TimKaine is a relentless optimist who believes no problem is unsolvable if you put in the work to solve it," Clinton wrote.
Kaine, who currently serves on the Senate Foreign Relations and Armed Services committees, could help Clinton bring in two key voting blocs: Hispanics and the battleground state of Virginia.
His Spanish language skills are a major asset in a year when the Latino vote could be decisive.
But the choice is unlikely to appeal to supporters of Clinton's former rival, democratic socialist Senator Bernie Sanders.
Kaine has praised the Trans-Pacific Partership trade deal, which Sanders passionately opposes and which Clinton came out against during her campaign.
He has also criticised regulations on big banks and is seen as being close to Wall Street.
Politically a middle-of-the-road Democrat, Kaine may help Clinton win votes from independent male voters.
The pick comes just days before the start of the Democratic National Convention in Philadelphia, where Clinton - and now Kaine - next week are to be elevated as the nominees to the party's presidential ticket.
A graduate of the prestigious Harvard Law School, Kaine has long been near the top of most lists of Democratic stars; he was mentioned as a running mate for Barack Obama in 2008. He endorsed Obama that year, angering Clinton supporters, but backed Clinton early this time around.
Although Obama instead picked Joe Biden as his number two, Kaine helped Obama turn Virginia from red to blue, the first time the state had chosen a Democrat for president since it voted for Lyndon B. Johnson in 1964.