Munster police and the Dusseldorf prosecutors' office said in a press release that officers searched a residence in the town of Castrop-Rauxel for "toxic substances" intended to carry out an attack.
The 32-year-old Iranian was "suspected of having prepared a serious act of violence threatening the security of the state by obtaining cyanide and ricin with a view to committing an Islamist attack", said investigators.
Another person was arrested during the operation and taken into custody, they added.
The main suspect will be presented in the coming days to an investigating judge ahead of possible pre-trial detention, they said.
Germany has been targeted in recent years by several Islamist attacks, including a 2016 truck attack on a Christmas market, that killed 12 people and left dozens injured.
A 13th victim died five years later having suffered serious injuries in the assault.
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