A beach has been cordoned off and bomb squads are on site after an explosive device was discovered.
The military team is working with emergency services and local agencies to keep the public safe while the wartime explosive device is dealt with in water off Elmer beach.
The large metal object, about 6ft long, was found in the water below the high water mark on Saturday morning (May 19).
A military ordinance disposal team attended and confirmed it is a German World War II sea mine, estimated to weigh about 1000 kilos.
Work to make the device safe is being carried out when the tides allow access and the area on the beach has been cordoned off, said Sussex Police.
A number of houses along the adjacent seafront have been visited and residents were advised to leave their windows open and to use only the rear part of the premises while the device is being dealt with and made safe.
No evacuations have been necessary at this stage, said police.
Chief Superintendent Jane Derrick said: “We are working to ensure that the public remains safe while this device is safely dealt with by the military ordinance experts.
“We are following their advice of safe distances and areas within which the public should not go.
“The situation is being kept under careful review and further action to ensure the public remains safe will be taken if necessary.
“In the meantime, we thank the local residents who have been involved so far for their cooperation and their patience.
“We ask other members of the public to avoid the area whether on the beach itself or swimming or sailing offshore.”
A spokesperson for the Maritime and Coastguard Agency said, “Coastguard teams from Littlehampton and Selsey are helping with a safety cordon and a HM Coastguard will be broadcasting to vessels to keep clear of immediate vicinity of Elmer Beach while military explosives team deal with the ordnance today.”