"This is a time of great challenges for America and I pray that the president will be successful in guiding our nation," Romney told supporters in Boston after calling Obama to congratulate him.
Obama defeated Romney in a number of key swing states, despite the weak economic recovery and stubbornly high unemployment that dogged his campaign.
Television networks called the election late on Tuesday, but the Romney campaign waited more than an hour to agree on the results in Ohio.
"I so wish, I so wish, that I had been able to fulfil your hopes to lead the country in a different direction, but the nation chose another leader," Romney said in his concession speech.
This is the second time the former Massachusetts governor has made a run for the presidency. Romney had promised to revive the nation's economy through reforming the tax code, reducing the debt and confronting China on its trade practices.