It came after several pieces of ordnance were located on the beach at around 11.25am on Saturday.
The Selsey Coastguard put a cordon in place and closed off the beach.
The Explosive Ordnance Disposal Team (EOD) was paged, but was on another job in Portsmouth and was unable to attend before the pieces were covered by the incoming tide.
Early the next morning, the team was retasked to Medmerry Beach.
A controlled explosion was carried out at just after 12pm and the beach was reopened.
A Coastguard spokesman said: “Ordnance comes in all shapes and sizes and ordnance found on this beach has been found to be highly explosive, so please do not touch it and call the Coastguard straight away.”