Earlier, Congressman Louis Gohmert said that he had contracted the coronavirus.
“Members of the house and staff should wear masks at all times in the house chamber,” Pelosi said, speaking in the chamber.
She noted that lawmakers can temporarily remove their masks when they are given the floor.
An employee familiar with the decision said Pelosi made it following a discussion with congressional physician Brian Monahan after Gohmert’s announcement.
At least two members of the House of Representatives reported that they went into quarantine after contacts with Gohmert.
Attorney General William Barr passed the COVID-19 test after testifying at a hearing on Tuesday that Gohmert attended.