North Korea said it had detained a 24-year-old American tourist earlier in April for what it said was "a gross violation of its legal order."
The announcement was made while US President Barack Obama was visiting South Korea, one of Washington's closest allies and still technically at war with Pyongyang.
"A relevant organ of the DPRK put in custody American Miller Matthew Todd, 24, on April 10 for his rash behavior in the course of going through formalities for entry into the DPRK to tour it," the country's official KCNA news agency said, using the country's official name, the Democratic People's Republic of Korea.
Kenneth Bae, a Korean American missionary, has been held for more than a year in North Korea and sentenced to 15 years of hard labor on charges of trying to overthrow the state.
North Korea rejected an offer for Robert King, the US special envoy for North Korean human rights issues, to visit Pyongyang to discuss Bae's case last August and rescinded an invitation for him to visit in February.
KCNA said Todd had a tourist visa for the DPRK, but tore it to pieces and shouted that he had come "to the DPRK after choosing it as a shelter."
Obama said on Friday that North Korea represented a threat not just to Asia but to the United States. He and South Korea's President Park Geun-hye said they would respond firmly to any "provocations."
(Reporting by Ju-min Park, additional reporting by Meeyoung Cho; Editing by Sonya Hepinstall and Angus MacSwan)