Pyongyang has detained a number of US citizens in recent years, using them as a tool to extract visits by high-profile figures, including former President Bill Clinton, for propaganda means.
North Korea periodically accuses the US of military hostility and conspiracy to overthrow its leadership. The two states have been locked in a tense diplomatic conflict over Pyongyang's nuclear and missile programmes.
The latest American to be held was being questioned by authorities for conduct inappropriate for the purpose of his visit as a tourist.
A State Department official said Washington was aware of reports a third US citizen had been detained.