Investigators suspect Ekaterina, 33, of maintaining telephone contacts and exchanging WhatsApp messages with off-duty police officer Mevlut Mert Altintas into late November, Turkish media said.
The Russian Embassy in Ankara has so far been unable to comment on the arrest to Sputnik.
Altintas shot Karlov at the opening of an art gallery exhibition on December 19, an act Moscow regards as a terrorist attack, and was killed by police at the scene.