Swimmers are being warned to be careful after a scaffolder working on Mingarry Castle, near Kilchoan, Lochaber, reported seeing the massive lion's mane jellyfish.
The species is one of the most dangerous in the world, known to have tentacles stretching up to 120ft.
In healthy adults, the tentacles can cause a painful red sting and become stuck to the skin, requiring careful removal afterwards. The sting has also been known to be deadly to people with underlying health problems and the elderly.
Several of the red and orange creatures have been seen in waters around Ardnamurchan by kayakers in recent weeks and a sighting was also made near Arisaig, both in Lochaber.
The lion's mane jellyfish also preys on the harmless blue moon jellyfish, which are normally common in west coast waters but which have been notably absent in Ardnamurchan this year.
NHS 24 medical director, Professor George Crooks, said: "If you are unfortunate enough to be stung, the most important thing to remember is not to rinse the wound in fresh water and not to rub it or apply ice - even if the area feels hot.
"Soak the affected area in vinegar for 15-30 minutes and, if vinegar is not available, use alcohol or sea water."