The suspect reportedly encouraged extremists to carry out attacks in Germany rather than travel to Syria, according to a report seen by German broadcaster ARD.
If confirmed, it is another blow to the country's security forces, who have already been criticised, in a recent report, for making serious errors.
The North Rhine-Westphalia police force in west Germany, where the informant worked, did not comment on the matter.
The informant allegedly had strong links to Tunisian Anis Amri, who killed 12 people and left 56 injured when he ploughed a truck into crowds in Berlin in December last year.
Officials have already admitted a series of security failures that allowed Amri to register under multiple identities and evade authorities while he was in contact with Islamist militants.
Several lawyers have said the alleged police informer tried to push their clients, accused of having jihadists links, to carry out attacks.
“Let's kill those non-believers. We need good men to do these attacks here in Germany,” he allegedly said, according to lawyer Ali Aydin.