Police have arrested a suspected perpetrator, but are very secretive about the incident.
Erol Önderoglu from the media rights group Reporters Without Borders (RSF) says that Arslan had investigated the corruption in the city.
Önderoglu condemns the murder and demands that those responsible be held accountable.
In the last two issues of Ses Kocaeli, Arslan has accused the city's mayor, a member of the ruling AKP party, of awarding contracts to people close to the party.
Arslan has previously been attacked and also investigated for his articles.
Turkey ranks 153rd on the RSF's latest worldwide press freedom index.