Effective immediately, in accordance with CDC guidelines and DoD direction, the Director of Administration & Management has directed that service members, federal employees, onsite contractor employees, and visitors, regardless of vaccination status, must wear a mask in an indoor setting in installations and other facilities owned, leased or otherwise controlled by DoD in the Pentagon Reservation. Further changes will be published in updated guidance for the Pentagon Reservation Health Protection Condition (HPCON). If you do not have a mask, DoD is prepared to provide one at Pentagon Reservation entrances.
You can read the guidance memo here.