Clinton bypassed more liberal candidates by opting for the 58-year-old former lawyer and governor of Virginia by opting for Kaine, who had emerged as the favourite for the vice president role in the last week.
She said he had "devoted his life to fighting for others" when she announced the appointment on Twitter.
Kaine said he was "honoured" to have been chosen as her running mate.
Just got off the phone with Hillary. I’m honored to be her running mate. Can’t wait to hit the trail tomorrow in Miami!— Senator Tim Kaine (@timkaine) July 23, 2016
Kaine is seen by many as a safe and experienced candidate. He endorsed Clinton for the presidency in 2014 and was an active campaigner for her during the primaries.
However, some of his stances such as his opposition to abortion and his advocacy for an Asian free-trade pact could rile some of Clinton's more liberal support base.
Clinton and Kaine will now hit the trail to campaign against the controversial Republican nominee and billionaire Trump, who secured the Republican nomination on Tuesday.
He accepted the nomination on Thursday, vowing to create a "safer" America if he wins the November election.
UK bookmakers believe Clinton, who is the presumptive candidate until it is officially announced, is the hot favourite to win.
A recent report from the Economist Intelligence Unit expects a Clinton presidency, with the Democrats winning a majority in the Senate.
However, either candidate will start from a position of weakness given their unfavourability with the public and a divided Congress, which will make it hard for either of them to implement their agenda.