Arturo Pierre Martinez, 29, of El Paso, Texas, said he entered North Korea by crossing the river border with China.
Details of how and when he got into the country were not immediately clear.
In his comments to reporters, Martinez strongly criticised the US for alleged human rights violations.
His comments come amid North Korea's own loud protests of a resolution in the United Nations that could open the door for its leaders to face charges of crimes against humanity for human rights violations, raising questions of whether Martinez was trotted out to the media for propaganda purposes.