The Mercy docked at the Port of Los Angeles in late March to serve as a hospital for non-coronavirus patients.
The facility was designed to remain a strict non-COVID-19 zone with protocols for testing patients and staffers. It was brought in to help ease the burden from Los Angeles-area hospitals dealing with the surge in coronavirus patients.
A spokesman confirmed that a medical treatment facility crewmember has tested positive for COVID-19. The crewmember is being isolated on board the ship and will be transferred to an off-ship isolation facility to self-monitor for severe symptoms.
The diagnosis does not affect the ship's ability to treat patients, the spokesman said.
The ship has about 800 active-duty doctors, nurses and medical staff members on board.
The USNS Mercy will be docked in the Port of Los Angeles until September or until it is no longer needed.
Last week a train engineer deliberately ran the locomotive off the tracks near the Mercy in an attempt to hit the ship and draw attention to what he felt were alternative motives to the ship's presence.
The suspect, Eduardo Moreno, remains jailed and held without bail on a federal count of train wrecking, holding a potential sentence of up to 20 years in federal prison.