Kim Sang-Duk, or Tony Kim, was detained at the capital's airport on April 22, as he tried to leave the country after teaching for several weeks at an elite university.
In the North's first confirmation of the professor's detention, the official Korean Central News Agency (KCNA) said he had been held for "committing criminal acts of hostility aimed to overturn the DPRK", using the country's official name.
It added that Kim was "under detention by a relevant law enforcement body which is conducting detailed investigation into his crimes".
Kim had been teaching accounting at the Pyongyang University of Science and Technology (PUST), which said last month that the professor had been held.
The school - founded by evangelical Christians from overseas and opened in 2010 - is known to have a number of American faculty members.