Won Moon Joo, who North Korea says has permanent residency in the US and lives in New Jersey, was arrested on April 22 after crossing the Amnok River from the Chinese town of Dandong, the official Korean Central News Agency said.
The 21-year-old man is being questioned by state authorities and has admitted that his actions were in violation of North Korean law, the agency said.
An official from South Korea's Unification Ministry, who didn't want to be named, citing office rules, said it couldn't immediately confirm whether Joo was a South Korean citizen and was being held in North Korea. Officials from South Korea's intelligence agency were unreachable for comment.
In March, North Korea announced that it had detained two South Korean citizens over alleged espionage. It has been holding another South Korean man since late 2013 on suspicion of spying and allegedly trying to set up underground churches in the North. He was sentenced last year to life in prison with hard labor.
Also last year, the North released three Americans two of whom entered the country on tourist visas and Kenneth Bae, a Korean-American missionary who was convicted of "anti-state" crimes. An Australian missionary detained for spreading Christianity was deported after he apologised for anti-state religious acts and requested forgiveness.