The South Korean news agency Yonhap said the detained American citizen, in his 50s, was a former professor from Yanbian University in China and had been in North Korea for a month in connection with relief programmes.
He was arrested at Pyongyang International Airport, Yonhap said.
In January last year, US student Otto Warmbier, 21, was arrested for trying to steal a propaganda sign from a hotel while visiting North Korea.
The US has in the past accused North Korea of detaining its citizens to use them as pawns.
After the brother of North Korean leader Kim Jong-un was murdered recently in Malaysia, Malaysian citizens in Pyongyang were arrested and held until North Koreans suspected of the murder were released in Kuala Lumpur.
Tension remains high on the peninsula following the North's recent failed missile test and massive military parade showing off its latest hardware.
North Korea has said it is ready to sink the Carl Vinson, and on Sunday said it would strike Australia with nuclear weapons if it remained an ally of the United States.
North Korea's aim is to develop nuclear weapons small enough to put on ballistic missiles, but there is no evidence yet it has done so, or that it has missiles with the range to reach long-distance targets.
This week US Secretary of State Rex Tillerson, on a tour of Asian nations, said the US was "reviewing all the status of North Korea, both in terms of state sponsorship of terrorism as well as the other ways in which we can bring pressure on the regime in Pyongyang".
North Korean media responded by warning of an unspecified "super-mighty pre-emptive strike".