Security officials were not immediately available to comment on who was behind the hijacking but Kocaeli Mayor Ibrahim Karaosmanoglu said an assailant told the crew he was from the PKK.
"The person hijacking the ferry said he was a member of the PKK and wanted to draw attention to it in the media," Karaosmanoglu told broadcasters.
Security forces were prepared for the possibility that the hijacker may want to take the vessel to nearby Imrali island in the Sea of Marmara, where PKK leader Abdullah Ocalan has been jailed since 1999.
"Don't intervene. I have a bomb. I will detonate it," it reported the hijacker as telling security forces.
Three coastguards and a helicopter were following the 400-passenger capacity ferry.
A series of airplane hijackings have been carried out in the
last two decades but they have been rare in recent years and hijackings at sea are even rarer.
In January 1996, pro-Chechen gunmen hijacked a Black Sea ferry with 200 passengers on board and threatened to blow it up in protest at a Russian attack on Chechen separatists. Those hijackers surrendered three days later.