Police arrested five suspects, including four women, as they carried out raids in more than 40 locations across the German capital, Berlin police and prosecutors said in a statement.
Authorities also raided homes in nearby Potsdam, Frankfurt and the eastern city of Goerlitz.
Investigators believe the suspects recruited women in Portugal, while in Nigeria, the gang charged men up to $15 000 for the scheme, with part of the money going to the Portuguese "brides".
The ring would then falsify documents such as marriage certificates.
Armed with the fake marriage certificate, the pretend couples would meet briefly in Germany to present themselves to authorities and apply for a legal, EU residency permit for the "husband", news agency DPA reported, quoting a police spokesperson.
"So far we have established more than 70 cases where marriages were faked to obtain EU residency permits," the police statement said.
Officers seized hard drives, mobile phones and ID documents in the raids, as well as 300 000 euros in cash, according to DPA.
German police said they worked with Europol and their Portuguese counterparts to smash the ring, and that searches also took place in Portugal.
Faced with an influx of more than a million migrants since 2015, German authorities say they are increasingly coming across cases of counterfeit marriages as it has become more difficult for migrants from Africa and Asia to claim asylum.