The citizen was named as 30-year-old Aijalon Mahli Gomes, from Boston, the Korean Central News Agency reported.
His "crime has been confirmed", it said, but did not provide further details.
300,000 North Koreans have fled to China risking their lives to flee the mass starvation and brutal oppression of the Stalinist North Korea Kim Jong regime.
The citizen was named as 30-year-old Aijalon Mahli Gomes, from Boston, the Korean Central News Agency reported.
His "crime has been confirmed", it said, but did not provide further details.