The announcement came minutes after donors and longtime Clinton associates received emails from Mrs Clinton’s campaign chairman John D. Podesta informing them of her candidacy.
It follows Mrs Clinton’s failed attempt at the 2008 election, when she was beaten for the Democratic nomination by now US President Barack Obama.
Mrs Clinton made her announcement via Twitter and a video on YouTube.
“I’m running for president … everyday Americans need a champion. And I want to be that champion,” she said. “So I’m hitting the road to earn your vote – because it’s your time. And I hope you’ll join me on this journey.”