Deryan Aktert, head of a local office of the ruling Justice and Development Party (AKP) in the Diyarbakir province, was hit by gunfire at a gas station and later succumbed to his injuries, the official told AFP on condition of anonymity.
It marked the second attack targeting ruling party officials within 24 hours, after the killing at the weekend of another ruling party official, Aydin Mustu, in the eastern province of Van.
Turkish police have detained at least 28 suspects, according to a security source.
The PKK, listed as a terror group by Turkey and its Western allies, has waged a bloody insurgency against the Turkish state since 1984.
The group has resumed attacks on Turkish security forces after the collapse in 2015 of a fragile truce.