Kaine, who backed Barack Obama when he beat Clinton for the Democratic presidential nomination in 2008, announced his support on Twitter and at breakfast meeting of the South Carolina Women's Democratic Council in Columbia, South Carolina.
"Today I encouraged Hillary Clinton to run for president & pledged my support for her candidacy if she does," Kaine said in a tweet that linked to his remarks on www.readyforhillary.com, website of one of the most prominent pro-Clinton organizations.
"She understands the challenges facing Americans from all walks of life and has the compassion and skill necessary to help improve our everyday lives," Kaine, former governor of Virginia, said in remarks prepared for the breakfast meeting, according to the website.
Kaine added Clinton's work as secretary of state, senator from New York and first lady during her husband Bill Clinton's presidency also give her a deep background on foreign policy.
Polls taken this year show Clinton with an advantage of more than 50 percentage points over other potential Democratic candidates, including Vice President Joe Biden.
Clinton said in April she was thinking about running in 2016 and was "going to continue to think about it for a while."
Last year Democratic Senators Claire McCaskill of Missouri and Charles Schumer of New York publicly announced their support for Clinton and the 16 Democratic women in the Senate also sent her a letter urging her to enter the race.