The German public prosecutor's office said on Thursday that seven men were arrested in the western state of North Rhine-Westphalia on suspicion of terrorism.
According to prosecutors, the suspects allegedly founded a terrorist group in Germany and were supporting the so-called IS group.
Police in the Netherlands also arrested two people, a man from Tajikistan and wife, on related terrorism suspicion.
The two suspects in the Netherlands were "in contact with several persons residing in Germany," said a statement from the Dutch Public Prosecution Service.
"The police investigation has led to the suspicion that the man is a member of IS and that he was given the order to plot a terrorist attack," the statement said.
"Although the plans were not yet concrete, they were serious enough for the Netherlands Public Prosecution Service to intervene," it added.