Mr Obama, a Democrat, said on his website he had formed an exploratory committee, which would allow him to raise money and hire staff for the run.
He is one of his party's rising stars, having electrified the 2004 Democratic convention with a powerful speech.
He opposed the war in Iraq and has a solidly liberal Senate voting record.
In a video announcement posted on his website, he said Americans wanted to move beyond partisan politics to find common-sense solutions to problems.
"Politics has become so bitter and partisan, so gummed up by money and influence, that we can't tackle the big problems that demand solutions," he said.
"We have to change our politics, and come together around our common interests and concerns as Americans."
Mr Obama, 45, has generated unusual excitement in Washington since he was elected in 2004, even being described as a "beach babe" after he was photographed visiting family in Hawaii.
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Born 4 Aug 1961 in Hawaii
Studied law at Harvard
Worked as a civil rights lawyer in Chicago
Elected to the US Senate in 2004
His 2004 convention speech touched on his personal history as the son of a Kenyan man and a white woman from Kansas, hailing America as a land of opportunity for giving his father a chance to study there.
He said he would make a formal announcement on 10 February as to whether or not he would run for president.
Mr Obama is considered one of few Democrats who could snatch the party's 2008 nomination from fellow Senator Hillary Clinton.
She has not yet declared her candidacy.
Former vice-presidential candidate John Edwards is thought to be the strongest Democratic contender to have declared he is running for president.