Mr Obama prevailed despite lingering dissatisfaction with the economy and a well-funded challenge by Mr Romney.
Mr Obama's margin of victory is not yet certain because two states have yet to report results.
Mr Romney, the former governor of Massachusetts, has called the president to concede the race.
With Florida still too close to call, Mr Obama has won 303 electoral votes to Mr Romney's 203.
In Boston, where his campaign was headquartered, Mr Romney congratulated the president in an emotional concession speech.
He said he and Vice-presidential candidate Paul Ryan had "left everything on the field" and had given their all in the campaign.
"This election is over, but our principles endure," he said. "I so wish that I had been able to fulfil your hopes to lead the country in a different direction."
The state of Alaska, where polls have yet to close, is expected to vote for the Republican.
Under the US constitution, each state is given a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes - by prevailing in the mostly winner-takes-all state contests - becomes president.
The popular vote, which is symbolically and politically important but not decisive in the race, remains too close to call.
On Tuesday, the president held the White House by assembling solid Democratic states and a number of important swing states such as Colorado, Iowa, Pennsylvania, Michigan, Minnesota, Virginia and Wisconsin. His narrow victory in Ohio, a critical Mid-Western swing state, sealed the victory.