The Environment and Environmental Health Department said a higher than normal number have been reported.
Some swimmers have received multiple stings.
The north coast of the island has been particularly affected, with several different species reported.
Marine study group Jersey Seasearch said these included the mauve stinger, compass jellyfish and a large, non-stinging, mushroom-shaped jellyfish.
Stewart Petrie, head of Environmental Health, said people should not touch any part of a jellyfish - whether alive or dead - as both the body and tentacles can deliver stings.
"There are far more jellyfish than might normally be expected and while they are highly unlikely to have serious consequences, stings can be painful, particularly for anyone who receives multiple stings," he said.
"Parents should keep an eye out for children who are not confident swimmers and may be alarmed if they are stung while swimming."
Dog walkers have also been warned to be aware of the risks.
The department said pouring sea water slowly over a sting should help to reduce the pain.
Warning signs are being erected at beaches in Jersey to warn swimmers and beach-goers about the dangers of jellyfish around the island.