The body of Aygün Mucuk was found on Friday morning by a cleaner at the gang's hideout in Wettenberg in the western state of Hesse. Police say they currently have no leads.
Thomas Hauburger, a local police investigator, said the Giessen Hells Angels gang leader had died from multiple gunshot wounds.
Police said that they currently had no leads to who committed the crime and that it was unclear whether Mucuk died in a shootout or was shot dead without retaliating. Investigators said they were trying to ascertain whether Mucuk was armed when he was shot, and whether an argument or fight had preceded the shooting.
"We are working on the basis that the homicide must have either taken place during the night or in the early morning," Hauburger said.
Hesse's police force on Friday secured the biker gang's hangout, usually frequented by 30 to 50 members, and stopped and checked vehicles passing through the area. Officials also questioned neighbors and appealed for witnesses.
Rivalry between Germany's biker gangs
The rivalry between the long-established Hells Angels gang from Frankfurt and its counterpart from Giessen, whose members are mainly of Turkish descent, has led to clashes and shootings.
This May, two gang members were shot and critically injured in a crowded square in Frankfurt. Police believe that incident was triggered by an argument between the different Hells Angels factions. A 55-year-old biker gang member was arrested a week later in Romania.
Two years ago, clashes between the Frankfurt and Giessen factions at a Frankfurt club led to gunfire. Five people were injured, including Mucuk. Clashes and gun violence among biker gangs has also been recorded this year in Heidenheim in Baden-Württemberg, Hamburg and Leipzig.
Hesse's investigative criminal police force (BKA) estimates that around 700 people are members of one of the four different biker gangs in the western German state. The gangs are known for committing drug-related offences and other criminal activity, often while working in red light districts or in the pub and nightclub bouncers' scene.