Barack Obama urged Scots to reject independence, saying the US would prefer to retain a "strong, robust, united" ally.
"The United Kingdom has been an extraordinary partner to us," he said on Thursday afternoon in response to a question on Scotland's forthcoming independence referendum. "From the outside at least, it looks like things have worked pretty well.
"And we obviously have a deep interest in making sure that one of the closest allies we will ever have remains a strong, robust, united and effective partner."
Obama stressed that Britain has been a close ally to the US for many years and hinted he has a vested interest in the outcome of the referendum.
But Obama said it was up to the people of Scotland to have the final say, despite calling for Britain to remain united.
"Ultimately these are decisions that are to be made by the folks there," he said.
The comments were made in a joint press conference hosted by Obama and Prime Minister David Cameron at the G7 summit in Brussels.
Scottish residents will vote on whether Scotland should become independent from the United Kingdom in September.
Pro-union campaigners immediately seized on Obama's endorsement, tweeting anti-independence takes of Obama's iconic campaign posters.