More than 2% of Portugal's population have emigrated in the past two years, since the country entered the worst recession in decades, officials say.
Jose Cesario, the secretary of state for emigrant communities, said up to 240,000 people had left since 2011.
Most were young, highly-educated people fleeing to Switzerland or the oil-rich former Portuguese colony Angola.
He said more would probably have emigrated had job prospects in Europe not taken a turn for the worse.
Portugal was long known as a country of emigration, says the BBC's Alison Roberts in Lisbon, but during the 1990s the outflow reduced sharply and immigrants flooded in, as the economy boomed.
In the current unprecedented crisis that process has been reversed - this time with many highly educated youngsters leaving, adds our correspondent.
But while in the 1960s most emigrants went to France, today they are more likely to head for Switzerland - where Portuguese now constitute the largest foreign community - or Angola.
"There's a very large increase in Portuguese emigration to Angola. We admit that in 2012 between 25,000 and 30,000 Portuguese left for Angola," Mr Cesario said, adding that the figure was between 5,000 and 10,000 higher than in the previous year.
The numbers heading for Mozambique were also growing fast, he added, while many Portuguese were still striking out for countries closer to home such as Germany, the Netherlands and the UK.
Only Europe's economic slowdown and the high level of unemployment in other EU nations had kept overall emigration from being far higher, the secretary of state said on Friday.