The service member is a Marine, officials said, and he tested positive for the disease Tuesday.
"The Marine is in isolation at his home and will undergo further assessment by health professionals," according to the release.
He last worked at the Pentagon March 13 and self-isolated when "an immediate family member" started to show coronavirus symptoms. When he too became sick, he contacted his assigned medical facility to make a notification.
According to DefenseOne, which first reported the case, the Marine worked in the service's Plans, Policies and Operations office, responsible for execution of policies related to Marine Corps deployments, structure and force employment.
The Marine's workspace in his Pentagon office is undergoing cleaning, and a "thorough contact investigation" is being completed to minimize exposure risk, officials said.
The news comes the same day the Pentagon elevated the health protection condition status of all military installations worldwide to Charlie, the second-highest level and one signaling substantial risk of sustained community disease transmission.
According to numbers released Wednesday, there are now 227 military coronavirus cases and 435 cases total within the Department of Defense.
On Sunday, the Defense Department announced the first death from COVID-19 within the Defense Department: a contractor based in Crystal City, Virginia, near the Pentagon.
Defense Secretary Mark Esper has since moved to elevate the HPCON status of the Pentagon itself to Charlie, severely restricting building access, closing some entrances and laying the groundwork for medical screening of entrants.
-- Hope Hodge Seck can be reached at firstname.lastname@example.org. Follow her on Twitter at @HopeSeck.