Prime Minister Gordon Brown said he was looking forward to working with the new US President.
In a statement, Mr Brown said: "I would like to offer my sincere congratulations to Barack Obama on winning the Presidency of the United States.
"I would also like to pay tribute to Senator McCain who fought a good campaign and has shown the characteristic dignity that has marked a lifetime of service to his country.
"The relationship between the United States and the United Kingdom is vital to our prosperity and security.
"Barack Obama ran an inspirational campaign, energising politics with his progressive values and his vision for the future.
"I know Barack Obama and we share many values.
"We both have determination to show that government can act to help people fairly through these difficult times facing the global economy.
"And I look forward to working extremely closely with him in the coming months and years."
