"I'm running for president," Clinton said in a video released on her official campaign website. "Everyday Americans need a champion, and I want to be that champion."
"Americans have fought their way back from tough economic times, but the deck is still stacked in favour of those at the top," Clinton said in her first 2016 campaign video.
After her defeat to Barack Obama in 2008, Clinton firmly said "no" when asked whether she would run for presidency again. However, her positions have evolved in the past years.
The Clinton camp has already signed a lease for a new office space in Brooklyn Heights, New York, as her campaign headquarters.
"I wanted to make sure you heard it first from me -- it's official: Hillary's running for president," said John Podesta, Clinton's campaign manager, earlier Sunday in an emailed statement to donors and members of Clinton's 2008 presidential bid.
Podesta said Clinton is on her way to Iowa to meet voters, adding that a formal rally will be held next month.