This idea has been around a long time, and even made its way onto an episode of the popular TV show Friends. However, we can say without hesitation that applying urine to a jellyfish sting isn’t going to help much (plus it’s pretty inconvenient at a public beach).
If you are stung by a jellyfish or a Portuguese Man o’ War (a close relative), rinse the area thoroughly with vinegar to prevent more venom from being released, then remove the tentacles with a tweezer (do not use your bare hands). Soaking the skin in hot water or under a shower (110 F to 113 F) for at least 20 minutes can help relieve the pain.
The pain of the sting should gradually subside over the next 24 hours. If the sting is severe and covers a large area, or causes nausea, vomiting, difficulty breathing, or pain in an unaffected area, seek medical attention immediately. A few species of jellyfish are very toxic and their stings can be dangerous, even fatal in some instances.