The incumbant will stay in office for the next four years after having won 303 instead of 270 electoral votes required.
US President Barack Obama has pledged "the best is yet to come", after a decisive re-election victory over Republican challenger Mitt Romney.
After a hard-fought campaign which highlighted America's political divide, Mr Obama pledged, as he did four years ago, to work with his opponents.
Mr Romney echoed that call for unity as he graciously admitted defeat.
Voters also left the Democrats in charge of the Senate and Republicans leading the House of Representatives.
In the electoral college, the state-by-state tally that determines US presidential elections, Mr Obama has won 303 electoral votes to Mr Romney's 206.
America's first black president sealed victory with a clean sweep of the most important swing states, including Ohio, Virginia and Pennsylvania.
Mr Romney could only snatch Indiana and North Carolina from his rival's 2008 grasp.
The final swing state - Florida - remains too close to call.
But the Democratic incumbent's lead in the popular vote count was much slimmer - he had 50.3% to 48.1% for Mr Romney.
Thousands of Obama supporters hugged and cheered in the Chicago convention centre where he delivered his victory speech in the early hours of Wednesday.
"We have picked ourselves up, we have fought our way back and we know in our hearts that for the United States of America the best is yet to come," said Mr Obama, 51.
He was returning to the White House "more determined, and more inspired than ever about the work there is to do", he said.
Mr Obama pledged to work with Republican leaders in Congress to reduce the government's budget deficit, fix the tax code and reform the immigration system.
He also offered to meet Mr Romney to discuss how they could work together.
The Republican admitted defeat with a brief speech shortly after midnight on Wednesday in Boston.
"This is a time of great challenges for America, and I pray that the president will be successful in guiding our nation," Mr Romney said.
The 65-year-old former Massachusetts governor urged politicians on both sides to "put the people before the politics".
But Mr Obama's second-term agenda will lie largely in the hands of the Republican leader of the House of Representatives, Speaker John Boehner, with whom he bickered bitterly during his first term.