David Keynes, also known in Turkey as Alpaslan Demir, holds the copyright to ByLock, an encrypted messaging app that has been banned in Turkey and across parts of the Middle East.
The Bylock app was used by FETO-linked coup plotters during the July 15 coup attempt in 2016 that claimed the lives of more than 250 Turkish citizens.
The Anadolu state news agency said Keynes arrived in Istanbul on June 9 and turned himself in to the police under a "repentance law" that grants more lenient treatment to people who confess to crimes.
Anadolu said he was put in pre-trial detention and charged with terror-related offences that could see him jailed for up to 15 years.
FETO and its US-based ringleader Fetullah Gülen orchestrated the defeated coup of July 15, 2016, in which 251 people were killed and 2,734 injured.
Ankara also accuses FETO of being behind a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police, and judiciary.
