Anyone travelling to the mainland by air, sea or land will have to prove they have had two jabs at least 15 days prior to arrival.
The changes were introduced by the Portuguese government and are likely to last until 11 July.
On 25 June Malta reportedly outlined plans to require unvaccinated Brits to quarantine upon entering the country this summer.
The moves comes following rising concerns over the Delta variant of the coronavirus.
However, fully vaccinated Brits will be exempt from having to show a negative PCR test on entry to the Portuguese island of Madeira, which will be added to the green ‘watch list’ on Wednesday (30 June) alongside the Balearics and Caribbean islands such as Barbados.
UK Foreign Office guidance states: "If you have travelled from the UK to mainland Portugal you must quarantine for 14 days in the place you are staying or at a place indicated by the Portuguese health authority, unless you can show you have been vaccinated with an EU approved Covid-19 vaccine at least 14 days prior to travel.
"You must be able to show you have completed the vaccination programme."
