The National Park Service now requires masks in all buildings and facilities.
The order extends to all employees, visitors, partners and contractors.
In addition to buildings, masks will be required in spaces where people are unable to physically distance – including narrow or busy trails, overlooks and historic homes.
"Wearing a mask around others, physical distancing, and washing your hands are the simplest and most effective public health measures to help stop the spread of COVID-19," said NPS Office of Public Health Director Captain Sara Newman. "Getting outside and enjoying our public lands is essential to improving mental and physical health, but we all need to work together to recreate responsibly."
The Washington Monument reopened in October after being closed for six months while D.C. weathered the arrival of the novel coronavirus, only to shut down again in December due to COVID-19 exposure.
More recently, the iconic monument was closed due to a "credible threat" following the Capitol riot.
For additional details, check the National Park Service website.