Psaki, 42, said she was last in contact with President Joe Biden on Tuesday, when she met him in the White House, where they were more than 6 feet apart and wearing masks. Biden, who is tested frequently, last tested negative on Saturday, according to the White House.
Psaki did not accompany Biden on his trip abroad to Rome this weekend for the Group of 20 summit and next Glasgow, Scotland, on Monday for a U.N. climate summit.
Psaki had planned to travel with the president but scrapped 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 (via PCR) for COVID on Wednesday, Thursday, Friday, and Saturday," Psaki said in a statement. "However, today, I tested positive for COVID."
Psaki said she is only exhibiting 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," Psaki said.
White House staff and others traveling with the president have been undergoing daily tests for COVID-19 since before departing Washington and are all fully vaccinated. Many officials have also received booster shots, due to the close-quarters environment and frequent travel associated with their work.
Biden got his COVID-19 booster on Sept 27, shortly after federal regulators approved the third dose for many Americans.
Biden has been accompanied on the trip by principal deputy press secretary Karine Jean-Pierre. Psaki said she would return to work in person at the conclusion of a 10-day quarantine and following a negative rapid test.