The arrest of a German national comes amid heightened tensions between Berlin and Ankara. Mesale Tolu has been in Turkish custody since the end of April, but German officials were not informed about the case.
The German Foreign Ministry has been demanding consular access to Mesale Tolu since it learned of the arrest only by way of media reports.
Martin Schäfer, the ministry's spokesman, called Turkey's failure to notify Germany "regrettable."
Foreign Ministry spokesman Schäfer said it was important that Germany could provide consular assistance to Tolu
Turkish authorities were obligated to handle Tolu's case "not only with decency, but according to international law."
"We will make this point loud and clear."
According to the Foreign Ministry, German diplomats have not been granted access to her - in accordance with the Vienna Convention on Diplomatic Relations - since her arrest on April 30.
The Foreign Ministry spokesman also stressed that Tolu no longer holds a Turkish passport according to official records since her reported naturalization as a German citizen in 2007.
According to Germany's ARD public broadcaster, the 33-year-old was being held in a women's prison in Istanbul. She has been accused of spreading "terrorist propaganda," as well as "membership in a terrorist group."
Similar charges have been brought against dozens of reporters and journalists in detention in Turkey since the July 15, 2016 failed coup.
It remains unclear whether Tolu was working in Turkey in a journalistic capacity, however. Her most recent employment contract was reportedly as a translator with the extreme left ETHA news agency.
Tolu has also reportedly written in the past for the Netherlands-based Firat News Agency (ANF), which the Turkish government accuses of having links to the outlawed terror group PKK (Kurdistan Workers' Party).
Rift between Germany and Turkey
The case could inflame tensions between the two countries once more. Ties between NATO allies Germany and Turkey have plunged to a low over President Recep Tayyip Erdogan's move to tighten his grip on power.
The arrest of German-Turkish journalist Deniz Yucel, who was jailed on terror charges in February, has also contributed to increasingly sour relations between the two states. In Yucel's case, it took Turkish authorities several weeks to grant consular access to the 43-year-old.
With the arrest of Mesale Tolu, the known number of German nationals in custody in Turkey has risen to six. Four of those in detention, however, are reportedly dual nationals of Turkey and Germany.
ss/kms (AP, AFP, dpa)