The president has an online network of millions of Americans and his web campaign was widely seen as a key plank of his election success in 2008.
The announcement was widely expected, and his campaign team are set to file election papers this week.
In an e-mail to supporters, Mr Obama said the campaign would start small and grow over time, "with people organising block-by-block, talking to neighbours, co-workers, and friends".
"So even though I'm focused on the job you elected me to do, and the race may not reach full speed for a year or more, the work of laying the foundation for our campaign must start today."
Unveiling a new look for Mr Obama's campaign website, his team told supporters: "The idea is to improve upon what's worked for the past four years, scrap what hasn't, and build a campaign that reflects the thoughts and experiences of the supporters who've powered this movement."
After the 'shellacking' A number of Republican presidential hopefuls are expected to seek the nomination to run against Mr Obama.
Mr Obama took office in January 2009 with near-record approval ratings, but has seen his support fall during his first two years in office.
In November 2010 Mr Obama's Democratic Party lost control of the House of Representatives and shed several Senate seats in mid-term elections to Congress.
Since WWII only two presidents have failed to secure a second term after serving a full first term
- Democrat Jimmy Carter, damaged by the Iran hostage crisis, was beaten by Ronald Reagan in 1980
- In 1992 Republican George H W Bush lost to Bill Clinton and his slogan 'It's the economy, stupid'
- In 1974 Republican Gerald Ford succeeded Richard Nixon, but was voted out in 1976
The president called the result a "shellacking" and vowed to listen to the concerns of US voters.Despite shedding some support, polling by the Pew Research Center suggests Mr Obama is in a stronger position at this stage than either Bill Clinton or George W Bush, both of whom went on to win re-election convincingly.
In polling carried out in March, some 47% of registered voters said they would like to re-elect Mr Obama, with just 37% saying they would vote for an as-yet-unknown Republican candidate.