"We've filed exploratory papers today, so the process is moving forward on that front," he told reporters Wednesday, his final day in office.
A spokesman for Romney later said the paperwork officially would be filed late Wednesday afternoon in Washington with the Federal Election Commission. The formation of an exploratory committee allows Romney to raise and spend money for a presidential run.
Romney's confirmation of his plans comes after a 10-day period of contemplation during a family vacation in Utah and follows several years in which he acknowledged he was considering a White House run but hadn't made a final decision about pursuing the presidency.
If elected, Romney would be the nation's first Mormon president.
The one-term governor joins a GOP field in which Sen. John McCain of Arizona and former New York City Mayor Rudy Giuliani have grabbed the lead in early polling. Both created exploratory committees late last year.
With McCain and Giuliani in the field, CBS News national political correspondent Gloria Borger says Romney "has to be the conservative in the race. I think that's what he going to try to portray himself as. He knows Giuliani is a Republican liberal and McCain is really out there on the war in Iraq, wanting to send more troops into Iraq. That's his big problem. So Romney wants to be the social conservative here."
Romney's problem, Borger says, is that "he's a Mormon. A lot of evangelical Christians say they will not support a Mormon for president."
Other Republican candidates include Kansas Sen. Sam Brownback and former Wisconsin Gov. Tommy Thompson, who have established exploratory committees. California Rep. Duncan Hunter and former Virginia Gov. Jim Gilmore have said they intend to follow suit. Others said to be mulling a bid include Arkansas Gov. Mike Huckabee, New York Gov. George Pataki, Nebraska Sen. Chuck Hagel, and former Oklahoma Gov. Frank Keating.
In recent weeks, Romney has faced questions about his conservative credentials on issues such as gay rights and abortion. Romney challenged Democratic Sen. Edward Kennedy in 1994, and in a letter he promised a gay Republican group he would be a stronger advocate for gays than his rival.
Romney ran as a moderate during his gubernatorial campaign. Despite saying he personally opposed abortion, he not only pledged to leave the state's abortion laws intact, but noted his mother, Lenore, ran for U.S. Senate in Michigan in 1970 as a supporter of abortion rights.
He now stresses his opposition to abortion in speeches across the country.
In 2002, Romney's supporters also handed out fliers with well wishes from him and his running mate at Boston's annual Gay Pride Parade. And he was endorsed by the Log Cabin Republicans, a group of gay party activists.
Nonetheless, Romney has insisted his opposition to gay marriage has been unflinching. He has lambasted the Supreme Judicial Court for its November 2003 decision making Massachusetts the first state to legalize gay marriage.
Not only does Romney diverge from conservatives on some social issues, but many evangelicals don't consider Mormons a true branch of Christianity.
Willard Mitt Romney, 59, is the son of former Michigan Gov. George Romney, who ran unsuccessfully for the presidency in 1968.
Mitt Romney received his B.A. in 1971 from Brigham Young University, and then went on to simultaneously earn degrees from Harvard Business and Law schools, graduating cum laude from law school and in the top 5 percent of his business school class.
Romney remained in Boston, where he helped found a multibillion-dollar venture capital firm and amassed a multimillion-dollar fortune. He rose to national prominence when he successfully resurrected the 2002 Olympic Winter Games in Salt Lake City, which were mired in a bribery scandal.
His only major failure was the unsuccessful campaign against Kennedy in 1994, although he performed better than many pundits had expected.
Romney was reared in Bloomfield Hills, Mich., an exclusive suburb outside Detroit. His father was chairman of American Motors Corp., and has been credited with coining the phrases "compact car" and "gas-guzzling dinosaur."
Romney sought and won the chairmanship of the Republican Governors Association, allowing him to spend the 2006 midterm election traveling the country and dispensing cash to his party's gubernatorial candidates.
He raised a record $20 million and was especially generous with RGA money in early presidential states like Iowa, Florida and his native Michigan.