All United Airlines flight attendants are now required to wear a face covering while working. United is the first major U.S. carrier to require masks.
Unions representing flight attendants have welcomed the new measure.
A spokesman for the Allied Pilots Association said: "Flight crews are a unique vector for the virus, traveling all over the country. It is vital that our crews be treated as essential workers and have access to personal protection equipment and testing."
But the union says it wants passengers to be required to wear masks too. Earlier this week, Canada began requiring face coverings for all passengers.
The change comes after reports that employees from American Airlines, JetBlue and FedEx were dying from the coronavirus.