Angela Hawke was walking her dog at Kingston Gorse Beach yesterday (Friday, October 16) when she spotted the dead creature.
She said: “I alerted the council and they are informing the beach officers.
“Fortunately, my dog is fine.”
Touching the Portuguese-man-of-war can cause a reaction, which in extremely rare cases can be fatal.
Three Portuguese-man-of-war, each about eight inches long, were also discovered on Shoreham beach this morning (Saturday, October 14).
Eight of the creatures washed up on nearby Sussex beach Bracklesham Bay in Chichester, prompting a warning from the district council for surfers and swimmers to take care.
Reports of Portuguese man-of-war near the Sussex coastline are very rare, but usually occur earlier in the year, a spokesman from Chichester District Council said.
There have been recent reports in places such as Cornwall and Wales, added the spokesman.
Anyone who spots a Portuguese man-of-war is advised to leave the water and report the sighting to the council.