The court gave Joerg Kretschmer, a shooting club member who owned 15 guns, a 21-month suspended sentence for leaving the pistol used by his son, Tim, and hundreds of rounds of ammunition unsecured.
"Weapons and ammunition were not separated sufficiently," Judge Reiner Skujat said.
Police had said 14 of the father's guns were stored in a safe as required by law, but the 9-mm Beretta used by Tim had been left in his father's bedroom.
Clad in black combat gear, Tim, 17, opened fire on students in a classroom at his former school in the southern town of Winnenden, killing nine students, three teachers, and three others before shooting himself dead when cornered by police.
Dozens of victims' relatives took part as plaintiffs in the prosecution of Kretschmer the father, 52, who had sought an acquittal and remained mostly silent in courtroom appearances.
Prosecutors had said he was probably aware of the fragile mental state of his son, who had been treated for depression before breaking off treatment.
The killings prompted calls for a tightening of gun laws in Germany that are already far stricter than in the United States, where such shootings are more common.