"Because the US has, relative to its 'AAA' peers, what we consider to be very large budget deficits and rising government indebtedness and the path to addressing these is not clear to us, we have revised our outlook on the long-term rating to negative from stable," S&P said in a statement.
But S&P said the US had until 2013 to come up with a credible plan for addressing its financial problems.
"We believe there is a material risk that US policymakers might not reach an agreement on how to address medium- and long-term budgetary challenges by 2013," it said.
"If an agreement is not reached and meaningful implementation is not begun by then, this would in our view render the US fiscal profile meaningfully weaker than that of peer 'AAA' sovereigns."