Portugal as is a European Union (EU) member state and also a member of the Schengen Area. Travellers coming from within the Schengen Area do not go through border control in normal circumstances. Citizens coming from any of the the four EU Member states not in the Schengen area (Ireland, UK, Cyprus and Croatia) need only an identity card to enter Portugal. In addition to their identity card, minors must also present authorisation from their parents to travel.
In addition to all EU countries, Visas are not required by visitors from the following countries for visit up to 90 days in a 6 month period:
Albania, Andorra, Antigua and Barbuda, Argentina, Australia, Austria, Bahamas, Barbados, Bosnia and Herzegovina, Brazil, Brunei, Canada, Chile, Colombia, Costa Rica, Dominica, El Salvador, Georgia, Grenada, Guatemala, Holy See, Honduras, Hong Kong, Israel, Japan, Kiribati, Macao, Macedonia, Malaysia, Marshall Islands, Mauritius, Mexico, Micronesia, Moldova, Monaco, Montenegro, New Zealand, Nicaragua, Palau, Panama, Paraguay, Peru, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Samoa, San Marino, Serbia, Seychelles, Singapore, South Korea, Switzerland, Taiwan, Timor-Leste (East Timor), Tonga, Trinidad and Tobago, Tuvalu, United Arab Emirates, United States, Uruguay, Vanuatu, Vatican City and Venezuela.
Visitors from the countries not listed above need a Schengen Visa to enter Portugal the countries needing Visa can be found here.
Under the terms of the Convention Implementing the Schengen Agreement, flights between Schengen states are considered to be internal flights and passengers do not need to obtain another Visa.