White House Press Secretary Jen Psaki says she has contracted Covid-19 and is experiencing mild symptoms.
Ms Psaki, 42, said she was last in contact with US President Joe Biden on Tuesday, when she met him in the White House. She said they were more than two metres apart and wearing masks.
Mr Biden, who is tested frequently, had negative results on Saturday, the White House said.
Ms Psaki did not accompany him on his trip to Rome at the weekend for the G20 meeting, before flying to Glasgow on Monday for the UN Cop26 climate summit.
She had planned to travel with Mr Biden but cancelled the trip just as he was set to depart for Europe after learning that members of her household had tested positive for Covid-19.
“Since then, I have quarantined and tested negative for Covid on Wednesday, Thursday, Friday, and Saturday,” Ms Psaki said on Sunday.
“However, today, I tested positive for Covid.”
She said she was only showing mild symptoms.
“While I have not had close contact in person with the president or senior members of the White House staff since Wednesday, and tested negative for four days after that last contact, I am disclosing today’s positive test out of an abundance of transparency,” Ms Psaki said.
White House staff and others travelling with the president have been undergoing daily tests for Covid-19 since before they left Washington and are fully vaccinated.
Many officials have also received booster shots because of the close-quarters environment and frequent travel associated with their work.
Mr Biden had his Covid-19 booster on September 27, shortly after federal regulators approved the third dose for many Americans.
He has been accompanied on the trip by principal deputy press secretary, Karine Jean-Pierre.
Ms Psaki said she would return to work after 10 days of quarantine and a negative rapid test.