The 72-year-old head of state was "asymptomatic" and isolating in the presidential palace in Lisbon, his office said in a statement.
Last Wednesday, he had spent a few hours in "administrative isolation" after a member of his entourage tested positive, but the president then tested negative and had not been placed in quarantine because his contact with the person concerned had been considered "low risk".
Portugal is facing a new lockdown after a record 122 deaths in the past 24 hours and nearly 4,000 people in hospital on Monday.
Prime Minister Antonio Costa said the Socialist government was preparing to announce on Wednesday restrictions similar to those imposed during the first lockdown in March.
The campaign for the January 24 presidential election, which officially began on Sunday, had already been cut to the bare minimum and will be suspended on the announcement of a new lockdown to curb the surge in new cases.
Polls predict the re-election of the conservative Rebelo de Sousa in the first round of a ballot with no candidate officially supported by the ruling Socialists.