The selection of Kaine, a self-described "boring" Virginian with a reputation for low-key competence, could appeal to independents and moderates, but it quickly angered liberal groups that object to his advocacy for an Asian free-trade pact.
Clinton made the announcement via Twitter and a text message to supporters after the first day of a two-day campaign swing in Florida. She called Kaine to tell him about 40 minutes before the announcement, and called US President Barack Obama shortly after Kaine.
"I'm thrilled to tell you this first: I've chosen Sen. Tim Kaine as my running mate. Welcome him to my team," she said in her text message, Reuters reported.
Kaine, 58, edged out two other finalists – Cory Booker, a US senator from New Jersey, and Agriculture Secretary Tom Vilsack, according to a Democratic source familiar with the discussions.
The former secretary of state will be formally nominated as the party's presidential candidate for the Nov. 8 election at next week's Democratic convention in Philadelphia. She leads Trump in many opinion polls.