Biden, who received a Covid booster last month, left Washington for the G-20 summit in Italy on Thursday.
“I last saw the president on Tuesday when we were sitting outside more than two meters away wearing masks,” she added.
She said she tested negative every day from Wednesday through Saturday before testing positive on Sunday.
“Thanks to the vaccine, I only experienced mild symptoms that allowed me to continue working from home,” said Psaki in her statement.
She said she would resume personal work after being quarantined for 10 days after a rapid negative test.