Former Secretary of State Hillary Clinton is to announce she is running for president on Sunday, various US media report.
Mrs Clinton is expected to make the announcement on social media.
The 67-year-old will then travel to Iowa and New Hampshire, two early primary contests in the 2016 race.
The former first lady previously ran for president in 2008 and is already considered a strong contender for the Democratic presidential nomination.
Mrs Clinton is expected to release a video outlining her campaign themes but will put off a large, formal speech.
Analysis, Anthony Zurcher, North America reporter
Hillary Clinton is ready to run. Are voters ready for her?
In 2008 she came close - oh, so close - to winning the Democratic nomination, falling to the once-in-a-generation candidacy of Barack Obama.
Despite her near success, however, her effort exposed some worrisome flaws - bickering among her advisors, a veil of guarded aloofness that was only occasionally lowered and the perception that she lacked an explanation for why she sought the top office.
With a Democratic field devoid of compelling alternatives, Mrs Clinton likely has a much clearer path to the Democratic nomination this time.
This gives her the luxury of time to find her footing on the campaign trail after a long hiatus.
Come next summer, however, a battle-hardened Republican candidate will be waiting. She came up short last time. Soon we'll learn what, if anything, she's learned from past mistakes.
She recently faced criticism over using a private email address to conduct official business as secretary of state and having personal staff decide what emails should be given over to public records.
Mrs Clinton was a politically active first lady during her husband's two terms as US president and then served in the US Senate for several years.
She lost out to President Barack Obama during the 2008 Democratic presidential primaries.
Mr Obama later named her as secretary of state, a position she held until 2013.
Two major Republicans, Ted Cruz and Rand Paul, have formally announced their campaigns, but many more are expected to run, including former Florida Governor Jeb Bush - whose father and brother were both president.
Another Clinton-Bush election - the first was won by Mrs Clinton's husband Bill in 1992 - would revive the debate in the US about powerful political dynasties.
The Democratic field looks more narrow than the Republican one, but Mrs Clinton could face former Maryland Governor Martin O'Malley and possibly Vice-President Joe Biden.
Many in the left-wing of the party are urging Massachusetts Senator Elizabeth Warren to run.
2016 runners and riders
Only Cruz and Paul have formally declared but here are some names to watch:
- early Republican frontrunner is Jeb Bush
- but New Jersey Governor Chris Christie could battle Bush for the party's centre ground
- darling of the Tea Party is Texas Senator Ted Cruz
- firebrand liberal Elizabeth Warren is championed by many in the Democratic Party
- libertarian Rand Paul has his supporters - and enemies - among Republicans
- Hillary Clinton will have learnt much from her failed campaign of 2008
