"For me and the entire government, it is highly alarming that freedom of the press and speech are being restricted again and again," she said after Turkey detained at least a dozen journalists and executives from the Cumhuriyet daily.
The arrests were "the latest example of this already very sad trend," Merkel said at a joint press conference with Swiss President Johann Schneider-Ammann. "We have great doubts that it complied with the rule of law."
Adding that the German ambassador had visited the Cumhuriyet newsroom Tuesday, Merkel said: "The journalists can be certain of our solidarity, just like all the others in Turkey who, under difficult conditions, are active for freedom of the press and speech."
She said the issue would "obviously play a central role" in talks on Ankara's long-standing but currently stalled bid to join the European Union.
The Cumhuriyet arrests came after Turkish authorities fired more than 10,000 civil servants at the weekend and closed 15 pro-Kurdish and other media outlets, the latest purge since July's failed military coup aimed at ousting President Recep Tayyip Erdogan.
Merkel's government was initially criticized for its reaction to the arrests after her head spokesman Steffen Seibert said that German leaders wanted to "express concern" about the actions taken against the journalists.
The former editor-in-chief of the Cumhuriyet newspaper, Can Dündar, said in an interview with Die Welt, published on Wednesday before Merkel's comments that the German government's reaction had been "truly weak".
“The reaction of the German government was truly weak, also in comparison to other Western partners of Turkey, like the reaction of the US,” said Dündar, who himself was sentenced to more than five years in prison in May, though he was released pending an appeal and then left for Germany.