Tyler Adams from Fareham was on the beach with friends when he saw the sea creature by the pier near to The Briny restaurant.
The 22-year-old said: ‘I was sat on the beach for about 20 mins before I noticed it. I think I was the first to see it because as soon as I walked to it people followed. It seemed to be dead but everyone was still very cautious for obvious reasons.
‘There were lots of people on the beach and they were taking pictures with it, it was bigger than a basketball!
‘It's not actually the first one I've seen in Portsmouth this summer. Three or four weeks ago I was swimming quite far out towards Spitbank fort and saw a live jellyfish, though it was much smaller.’