Four of those on board the helicopter have been rescued, according to officials.
US Coast Guard officials said they responded to a downed aircraft report at about 11:00.
In a bizarre coincidence, the incident came a day after another US military helicopter crashed in the county of Norfolk, England, killing four people.
The aircraft in Wednesday’s incident went down 18 nautical miles east of the Virginia coast. The cause of the crash is not yet known.
Petty Officer David Weydert said that the missing fifth person was still being sought.
The Coast Guard cutter Shearwater and two MH-60S helicopters are taking part in the search.
The condition of the four rescued personnel has not been given.
The downed helicopter, assigned to Helicopter Mine Countermeasures Squadron 14 (HM-14), was on a routine training mission, the commander of Naval Air Force Atlantic said in a statement.
Norfolk is an important hub for the US military, and is home to Naval Station Norfolk, the largest naval complex in the world.
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