In a message posted on Twitter Monday, the head of the EU's executive arm said she took part in a meeting last Tuesday that was attended by "a person who yesterday tested positive." Von der Leyen was on a two-day trip to Portugal last Monday and Tuesday.
She said she tested negative for the virus on Thursday and that she will undergo another test later Monday.
Also on Monday morning, Research Commissioner Mariya Gabriel also announced she was self-isolating after a member of her team had tested positive.
"Pending test results, myself and my whole team are self-isolating in line with public health protocols and working from home. I feel well and have no symptoms," Gabriel said in a Twitter message.
Von der Leyen, who attended a two-day summit of EU leaders in Brussels late last week, said she will be in quarantine until Tuesday morning but feels well. Her isolation will keep her close to work, however: She has a small living quarters next to her office in the EU headquarters in Brussels.
Von der Leyen took part last Tuesday in a special meeting of Portugal's state council in the coastal town of Cascais, on the outskirts of Lisbon. State councilor Antonio Lobo Xavier tested positive for the virus on Sunday.
The meeting was attended physically by 15 of the 19 members of the president's advisory body.
Portugal's president, prime minister and the parliamentary president and five state councilors were tested on Sunday and were negative, media reported. More tests are taking place on Monday.
Two weeks ago, EU Council President Charles Michel was forced to postpone a summit of EU leaders because he was quarantining.