Ahmet Turk, 74, who was first elected in 1973 to represent Mardin in the national parliament and served as a lawmaker until 2015, was detained at home in what the state-run Anadolu agency said was part of an "ongoing terror investigation."
The authorities also detained Emin Irmak, the co-mayor of a district within Mardin. Both were stripped of office last week by the government in a crackdown which has seen at least 34 elected mayors removed from municipalities in the largely Kurdish southeast over suspected militant links.
Sources said seven other local administration officials in the region were also detained.
The leaders of the main pro-Kurdish opposition Peoples' Democratic Party (HDP) were arrested two weeks ago, drawing strong international condemnation of a widening crackdown on dissent under President Tayyip Erdogan.
Turkey has been fighting an armed Kurdish insurgency in the southeast and the government accuses the HDP and other Kurdish politicians of links to the PKK militant group, deemed a terrorist organization by the European Union and United States.
Thousands of officials from the HDP, parliament's second biggest opposition party, have been detained in recent months.
The PKK launched an insurgency against the state in 1984 in which more than 40,000 people have been killed. The HDP and other Kurdish parties deny direct ties with the group and say they are working for a peaceful resolution of the conflict.
The crackdown on pro-Kurdish politicians has run parallel with a purge of people accused of ties to US-based businessman Fethullah Gulen, accused by Turkish authorities of masterminding July's coup attempt. Gulen denies involvement.
More than 110,000 people have been sacked or suspended in the military, civil service, judiciary and elsewhere, while 36,000 people have been jailed pending trial as part of the investigation into the failed putsch.