The Smithsonian says it will require all visitors over the age of 2 to wear masks in indoor spaces and museums, regardless of whether they are vaccinated.
The policy is set to take effect Friday, the same say the National Air and Space Museum is scheduled to reopen, the Smithsonian announced.
The change comes after the Centers for Disease Control and Prevention updated mask guidance and advised people to wear masks in places where COVID-19 is surging.
D.C. has "substantial" community transmission of COVID-19, so masks should be worn indoors, according to the CDC.
Smithsonian museums began to allow visitors in May, then stopped requiring vaccinated people to wear masks about a month ago.
The masking requirement will apply to sites in D.C. and New York.
Ten Smithsonian museums plus the National Zoo are currently open to the public in D.C.