People taking part in the practice known as "tombstoning" are risking their lives, police have warned.
Sussex Police said there had been reports of people jumping from cliffs, ledges and other high structures into water to cool off in the hot weather.
There have previously been deaths as a result of the tombstoning craze in other parts of Britain.
A spokeswoman for Brighton and Hove Police said: "Tombstoning is a life-threatening pastime."
She added: "It is very hard to judge the depth of water due to swell and sea conditions. Serious injuries are often sustained when hitting the seabed or an underwater obstruction.
"Each year emergency services are called to deal with people who have injured themselves after jumping into the sea from Brighton's Palace Pier or the walls of Brighton Marina.
"It is only a matter of time before someone is seriously hurt or killed."
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