A 250-kilogram German bomb and a 50-kilo American one were discovered close to each other in an industrial zone of the town of Gießen in central Hesse state.
The discovery came Monday afternoon during construction work, which is often how World War II bombs in Germany are found.
Police said the German bomb had to be detonated as it was too degraded to be defused, while the US explosive could be safely deactivated.
Around 2,500 people, including 800 refugees at a reception centre, were briefly evacuated from their homes before the explosion, police added.
A total of 60 police, 60 firefighters, and 120 Red Cross members were deployed , according to the city.
Nearly 75 years after the end of the war, Germany remains littered with unexploded ordnance, a legacy of the Allied bombing campaign against Nazi Germany.
On Sunday a bomb likely dating from World War II exploded in a field in Limburg, also in Hesse, leaving a huge crater but causing injuries.