German reporter Billy Six, 32, is being held in Venezuela, the German foreign ministry has confirmed.
Venezuelan rights group Espacio Público says Mr Six, who writes for right-wing newspaper Junge Freiheit, has been charged with espionage and rebellion.
Relatives say he was arrested more than three weeks ago while reporting on Venezuela's economic crisis and the mass migration it has triggered.
Diplomatic officials have reportedly not been allowed to see him.
Rights groups say he is being held in the infamous Helicoide prison in the capital, Caracas, which has seen a number of prison riots this year.
In October, the prison again hit the headlines when jailed opposition councillor Fernando Albán died in custody.
The government alleged that he had killed himself by jumping from the 10th floor of the building, which serves as headquarters for the state intelligence service Sebin.
Opposition groups say he was killed and his body thrown from a window to cover up the murder.
You may also be interested in:
Mr Six was in Venezuela "a investigating drug trafficking activities, smuggling of fuel and strategic goods, human trafficking and even the exodus of Venezuelans" Gerardo Morón of Venezuelan rights group Espacio Público said.
His father, Edward Six, said he had been arrested in his hotel room on 17 November.
According to Edward Six, the Venezuelan authorities are using a photo his son took of President Nicolás Maduro at a rally in Caracas as evidence that he "violated a security zone". The journalist's father says the photo was taken from behind a security perimeter.
"He never touched a weapon, never joined in any demonstrations," Edward Six told Associated Press news agency. "He just was on the street. He talked to all these normal people. He asked them questions and put that on the internet."
The Venezuelan government has not yet officially commented on Mr Six's detention.
It is not the first time Mr Six has been detained. In December 2012, he was arrested by the Syrian army for entering the country illegally and held for three months before being released.