Mardin Metropolitan Mayor Ahmet Türk has been detained as part of the ongoing terror probe on links between the PKK terrorist organization and the Peoples' Democratic Party (HDP) municipalities.
Türk was suspended from duty on Thursday, and was detained along with Artuklu district Mayor Emin Irmak on Monday.
According to reports, counter-terror police detained both mayors in their houses.
Earlier this month, 13 lawmakers from the HDP were arrested. 10 of them, including co-chairs Selahattin Demirtaş and Figen Yüksekdağ, remain in custody awaiting trial on terrorism-related charges.
In October, the DBP co-mayors from Diyarbakır, the largest city in the southeast, were arrested and jailed pending trial on terrorism charges.
The PKK - listed as a terrorist organization by Turkey, the United States and EU - resumed its decades-old armed campaign in July last year and has been responsible for the deaths of more than 1,100 security force members and civilians, including women and children.
Around 10,000 PKK terrorists have been killed or apprehended in counter-terror operations throughout the country in the same period.