"The US government congratulates acting President Goodluck Jonathan," US ambassador to Nigeria, Robin Sanders, said in a statement in Abuja.
"We believe that the principles of democracy have been served well in Nigeria through the leadership shown by the National Assembly, the Governors? Forum, several ministers, and the courts in finding a way out of the political impasse," she said.
The parliament and the governors of all 36 states this week voted that Jonathan be made acting president in the absence of Yar'Adua, hospitalised in Saudi Arabia since November 23 for an acute heart condition.
"The best interests of the country and the future of Nigeria have been well served by this action," Sanders said.
She said "the United States stands ready to support the government and the people of Nigeria in achieving many important goals in its immediate future, including free and fair national elections in 2011."
An accurate, credible and transparent national voters roll would be a good first step, she said.
A gubernatorial poll held last week in southeastern Anambra State, witnessed by US officials, was flawed by late delivery of election materials and errors in the voters' register.
That election was seen as a test-run for next year's presidential poll in Africa's largest democracy.
US Johnnie Carson, on Tuesday ended a three-day official visit to Nigeria, during which he met Jonathan, the leadership of the parliament and some top government officials.,