The Stuttgart state court gave Joerg Kretschmer a 21-month suspended sentence for not securing his 9mm Beretta pistol, the news agency DAPD reported. The man's 17-year-old son used the weapon to kill nine students and three teachers at his former high school and three people elsewhere in March 2009.
The son, Tim Kretschmer, then killed himself with the weapon during an exchange of gunfire with police.
Judge Reiner Skujat said that while the father was not personally responsible for the horrendous crime committed by his son, the "defendant had completely failed" because he had let his son have access to the weapon.
The father had kept the gun unsecured in his bedroom in violation of German law. His other 14 firearms were properly secured.
The judge also said Kretschmer's parents should have been aware of their son's "potential for danger," because psychiatrists had informed them before the shooting that he was suffering from mood swings and had fantasized about killing people.
Kretschmer showed no emotion as he listened to the sentencing. He had not attended earlier court sessions, because of death threats from victims' relatives, the judge said.
Forty-three family members of the victims of the attack had joined the trial as co-plaintiffs, as allowed under German law.
Kretschmer's defense lawyer said he would appeal the court decision.
The attack in Winnenden in southwestern Germany was the nation's second-worst school shooting. A 2002 shooting spree in Erfurt left 17 dead, including the gunman.
Germany moved to tighten checks on gun owners after Kretschmer's attack .