They swim very slowly and are swept toward the coast by the currents in spring and summer. If you touch a jellyfish, even if it is dead, it can inject a substance into your skin that produces a burning sensation. Contact with the tentacles or parts of them, both in the water and on the sand, can cause injury.

Yes, jellyfish are common in the waters off Barcelona's beaches, particularly during the warm summer months, and many of them do possess a sting. The most frequent species encountered is the Pelagia noctiluca (Mauve Stinger), which is small and translucent-purple. Its sting is typically painful and causes a burning sensation, redness, and sometimes minor welts, but it is rarely dangerous to healthy adults unless they have an allergic reaction. Local lifeguards often use a "jellyfish flag" (usually yellow or white with two jellyfish icons) to warn swimmers when a bloom is present. If you are stung, the standard advice is to rinse the area with seawater (never fresh water, which can trigger more venom release) and remove any tentacles with a plastic card. While they are a nuisance, the jellyfish in the Mediterranean are significantly less dangerous than the life-threatening "box jellyfish" found in the Pacific or Indian oceans.