"I want to confirm that yesterday a fully vaccinated White House official tested positive for COVID-19 off campus," Psaki said, adding that the official, who has not been identified, had mild symptoms.
She said that in accordance with "rigorous" protocols, the official is staying away from the White House pending more testing, and that contact tracing had been carried out.
Psaki said there had been other breakthrough cases at the White House, but did not confirm how many or when.
"We know that there will be breakthrough cases. But as this incident shows, cases in vaccinated individuals are typically mild. The White House is prepared for breakthrough cases with regular testing," Psaki said.
In a similar case, the office of House Speaker Nancy Pelosi confirmed that one of her senior spokespeople had tested positive, after meeting with Democratic state lawmakers from Texas who had flown to Washington last week.
According to a statement by Pelosi's deputy chief of staff Drew Hammill, the person had no contact with the Speaker.
After making enormous progress against the pandemic, the vaccination campaign in the United States has stalled in recent weeks.
About 68% of adults have received at least a first dose, but there are large geographical disparities.
Opposition to the vaccine is significantly tied to the political divide in the country, with hostility much more pronounced among conservatives, particularly among supporters of former president Donald Trump.