Ahmet Turk was detained along with Emin Irmak, co-mayor of central Artuklu district in Mardin, as part of a "terror probe", the official news agency Anadolu said.
Before the latest detentions today, the pro-Kurdish People's Democratic Party (HDP) said that so far, 53 mayors have been dismissed and 39 mayors arrested in the region.
Most of the arrested mayors are from the Democratic Regions Party (DBP) which is closely allied to the HDP.
Meanwhile, Van co-mayor Bekir Kaya was placed under arrest on Friday, charged with "knowingly and willingly helping" the outlawed Kurdistan Workers' Party (PKK).
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Last month, the two mayors of the biggest Kurdish-majority city of Diyarbakir, Gultan Kisanak and Firat Anli, were detained and charged with belonging to the PKK.
Ahmet Turk had already been removed from his post last week and replaced on a trustee basis by Mardin province governor, Mustafa Yaman.
The state of emergency put in place after the July 15 attempted coup allows the authorities to sack elected mayors and replace them with state-appointed trustees.
But critics complain that the authorities are using the state of emergency to crack down on all opposition beyond the suspected coup plotters themselves.
The PKK has waged an insurgency inside Turkey since 1984. Violence renewed in July 2015 after the collapse of a two-and-a-half year ceasefire.
The group is proscribed as a terrorist organisation by Washington and Brussels.