If you’re traveling to Portugal with a Visa or Mastercard on hand, you’ll be covered almost everywhere in the country. You’ll be good with an American Express card in some places, too. Discover cards are accepted only in places with the Diners Club logo. But there are only a few.
That said, it’s a good idea to also carry some cash as some smaller business might not accept credit cards or have a minimum paying limit (usually 10 or 20 EUR). Keep around 75 to 100 EUR in cash to pay newspaper vendors, cafés, restaurants, bars and small souvenir shops.