Obama locked up a presidential campaign, defeating Republican rival Mitt Romney, though final Electoral College and popular vote numbers were not available.
A number of media outlets called the race for the president after reviewing the polling trends in critical remaining swing states.
A looming clash over US budget measures is now set to become the focus of investor attention following the re-election of Barack Obama.
President Obama needs to secure a deal with the US Congress to avoid the so-called "fiscal cliff".
This refers to the $600bn (£375bn) of tax rises and spending cuts that are due to hit the US economy in January.
US shares fell more than 2%, but this was also due to renewed concerns about the Euro zone economy.
Following news that the European Commission had downgraded sharply its growth forecast for the EU and Euro zone, the main US index, the Dow Jones, closed down 313 points, or 2.4%, at 12,933, its lowest level since early August.
The UK's FTSE 100 closed 1.6% lower, and Germany's Dax lost 2%.
"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," President Barack Obama has been re-elected to a second term, defeating Republican challenger Mitt Romney. America's first black president secured the 270 votes in the electoral college needed to win the race. Join the World Service for unprecedented live coverage of the latest analysis and reaction to Obama’s victory.
Matt Wells travels to Capitol Hill to hear whether America's Catholics - who make up around a quarter of the electorate - will follow their 'Catholic conscience'; he will ask why President Obama has gambled with the vote of such a huge number of people or whether the church has got it wrong and Catholic voters won't necessarily go with the advice of the church.