The two aircraft crashed in the skies above the town of Philippsburg, close to the French border, a German police spokesman said, adding that he was unable to say if there may be other victims.
Both aircraft went down between the town of Rheinhausen and Erlich lake in Baden-Württemberg and the site of the accident is reportedly near a gravel plant.
The cause of the collision also remains unclear, and the area where the aircraft came down has been cordoned off.
It also remains unknown whether the plane and the helicopter collided in the air over inhabited or uninhabited areas.
Collisions between aircraft in the sky are rare. Usually in accidents in the skies only one aircraft is affected. In a case in 2002 in Germany, a freight plane struck a passenger flight at high altitudes, killing dozens of people.