The two countries imposed the restrictions on Jan. 28.
The interior ministry said the restrictions would remain in place for a further 11 days, adding that the measures did not prevent citizens or residents of Portugal from entering the country.
Vehicles transporting goods, emergency vehicles and seasonal border workers are also exempt.
Nationals from Britain, Brazil and South Africa - where more contagious coronavirus variants were first detected - or any country with an infection rate above 500 cases per 100,000 people must quarantine for 14 days if they have entered via the land border, the statement said.
Portugal, which has so far reported 780,322 COVID-19 cases and 16,879 deaths, is slowly lifting some other restrictions after a three-month nationwide lockdown came into force early this year.
Meanwhile, Greece said on Sunday it was extending restrictions on domestic flights until April 12 and on international flights until April 19 as the number of new infections there continues to rise.
