Portugal is the latest euro zone economy to have a credit ratings downgrade.
Standard & Poor's on Wednesday cut Portugal's long-term rating to "A+" from "AA-", citing slow economic reforms and deteriorating public finances.
The agency has already cut its ratings for Greece and Spain and put Ireland on negative watch.
S&P said Portugal faces tough times ahead as the global economy deteriorates, especially in boosting a growth rate that has been near the lowest in Europe for half a decade.
Prime Minister Jose Socrates said last week a downgrade of the rating would be unfair.