Joe Robinson, 25, fought alongside the People's Protection Units of Syrian Kurdistan (YPG).
Turkey is hostile to the YPG because of its links with the Kurdistan Workers' Party, which is fighting for Kurdish autonomy in Turkey.
Mr Robinson, who used to live in Accrington, Lancashire, was detained with his girlfriend and her mother while the trio were on holiday in Turkey.
The Kurdistan Solidarity Campaign said previously: "Unfortunately, Joe has gone on holiday to Turkey not realising the kind of state it has now become.
"Turkey unfortunately continues to criminalise the Kurdish question and his jailed tens of thousands of Kurdish people, including elected MPs and mayors simply for peacefully campaigning for Kurdish rights."
"We stand ready to provide consular assistance to a British national in Turkey," the Foreign Office said.