Iran topped the list, with 25 percent naming it when asked which country is the greatest U.S. enemy, according to the Gallup Poll. Iraq came next at 22 percent, then China with 14 percent.
North Korea was named by 9 percent half the number who cited it a year earlier. Pyongyang agreed last year to disable its nuclear facilities in exchange for aid, though disputes have continued over implementing the deal.
China, a growing economic rival of the U.S., was cited by 11 percent as top enemy a year ago.
Gallup first asked the question in early 2001, before the terrorist attacks of Sept. 11. At the time, Iraq was seen as the biggest foe, followed by China and Iran.
Iraq has remained one of the most frequently named ever since, even though Saddam Hussein was overthrown and the current Baghdad government is backed by the U.S.
Republicans are more than twice as likely as Democrats to see Iran as the top U.S. enemy, while Democrats are likelier to name Iraq. Older people and those who say they closely follow world news are less likely to cite Iraq than the younger and less informed.
No other country was named by more than 3 percent.
The poll was conducted from Feb. 11-14 and involved telephone interviews with 1,007 adults. The margin of sampling error was plus or minus 3 percentage points.