The North's state-run KCNA news agency said the person entered North Korea as a tourist and "was caught committing a hostile act against the state," which it said was "tolerated and manipulated by the US government," Reuters reported.
The Korean-language KCNA report said the detainee was a Virginia university student and had entered the country with an "aim to destroy the country's unity". It did not elaborate.
An official at the US embassy in the South Korean capital Seoul said it was aware of the reported arrest.
A South Korean-born Canadian pastor was arrested in North Korea last year and given a life sentence for subversion. Earlier this month, a Korean-American man said in Pyongyang that he was being held by the state for spying.
South Korea warned that the United States and its allies were working on further sanctions to inflict "bone-numbing pain" on the North after its latest nuclear test this month, and urged China to do its part to rein in its neighbour.