David Cameron said he was "delighted to continue working together" with Barack Obama as the US president took the oath of office for a second White House term.
The Prime Minister offered his congratulations via Twitter after Mr Obama delivered his inaugural address in a day of pomp and ceremony in Washington.
"Congratulations to @BarackObama on his second inauguration. Delighted to continue working together," said his message.
Mr Obama urged his country to "seize the moment" as it emerged from a decade of war and deep economic woes.