After the initial discovery, the police have also found what they believe to be the body of the victim.
German police have arrested a suspect after a severed human head was left outside a courthouse in Bonn.
The gruesome discovery was made on Tuesday evening outside the city's district court. Police later said they found a body a few hundred metres away on the banks of the Rhine river.
Initial investigations suggest that both the victim and suspect — a 38-year-old man known to police for drug offences — were both homeless individuals.
"At around 17:35 (CET), witnesses informed the police control centre that a human head was lying in front of the Bonn court building on Wilhelmstraße," a police statement read.
"After the first arriving police forces confirmed this initial report, a suspect was found in the immediate vicinity and arrested."
"Further investigations led to the discovery of a corpse in the area of the old customs house a short time later."
Authorities have immediately opened a homicide investigation and are appealing for witnesses.