In announcing her intention on her Web site -- the words "I'm in" topped the page -- Clinton said she would seek voter input into the "bold but practical changes we need to overcome six years of Bush administration failures."
Clinton, 59 and the wife of former U.S. President Bill Clinton, said she would conduct a series of Web chats in the next few days on a variety of issues such as healthcare, education and retirement security.
"Only a new president can renew the promise of America," she wrote. "And only a new president can regain America's position as a respected leader in the world."
Clinton raised the number of Democrats vying for the nation's top job to seven.
Also seeking the party's nomination are U.S. Sen. Joseph R. Biden Jr. of Delaware, U.S. Sen. Christopher J. Dodd of Connecticut, former U.S. Sen. John Edwards of North Carolina, U.S. Rep. Dennis J. Kucinich of Ohio, U.S. Sen. Barack Obama of Illinois and former Gov. Tom Vilsack of Iowa.
New Mexico Gov. Bill Richardson is expected to announce soon that he is forming his exploratory committee.