The 1,901 counterfeit 200-euro banknotes were seized in an operation in the northern city of Oporto, police said in a statement.
A 46-year-old foreign man was arrested, police added, but neither his identity nor his nationality have been revealed.
The counterfeit notes were "of exceptional quality", officials said.
They are thought to be of a type that has been in circulation for a decade, with specimens turning up mostly in Spain, Italy, Germany, Bulgaria and Portugal.
This, police said, is the largest batch found so far.
The BBC's Alison Roberts in Lisbon says it is the second seizure of counterfeit euros this month in Oporto.
Police recently dismantled a suspected ring composed of five Portuguese citizens, accused of counterfeiting and passing off 50 and 20-euro notes.
That operation resulted in the seizure of fake notes worth 30,000 euros.
Portuguese police said the euro's growing importance as an international currency had made it an attractive forging target for organised crime in the European Union and beyond.
