The court in Leipzig decided on Monday that the social worker had to take some responsibility for the death of the toddler in June 2012, after his mother, 26, died.
Judge Elke Kniehase fined the man €3,600 for involuntary manslaughter through neglect, ruling that social services had not done enough to protect the boy.
As someone who was supposed to supervise the mother, the social worker had known of the risks of her long-term drug addiction, the court heard.
Since the start of 2012 there had been signs that the mother was losing control and taking hard drugs.
She stopped her methadone programme and gave her son to a childminder. But for two months before the deaths, the social worker had made no contact with the woman as she had said she wanted to move to Baden-Württemberg.
At a hearing in January, the social worker denied responsibility for the boy’s death and his defence had asked for an acquittal.
But the judge largely agreed with the state prosecutor’s argument.
The judge said that the drug-addicted mother was largely to blame for the tragedy, but added the social worker, who had lots of experience of working with drug addicts, should have been aware of the potential problems.
Around 30 colleagues followed the social worker’s trial in the courtroom and wore red badges in a display of support for him.