Christopher Bee was walking near Boscombe Pier when he came across the animal, believed to be a barrel jellyfish, measuring around 30 inches across.
The semi-retired landlord, who has lived in Bournemouth for three decades, walks along the beach every morning and regularly keeps an eye out for wildlife.
But he said he had never seen a jellyfish quite like the one he discovered stranded at the water’s edge.
Chris said: “I couldn’t believe the size of this enormous jellyfish.
“It’s very unusual, I have never seen one this big before.”
Jellyfish (Image: Christopher Bee)
He recalled the jellyfish was “a good couple of foot, maybe even more” across as it lay on the sand near the pier.
The species is believed to be a barrel jellyfish (Rhizostoma pulmo), also known as a “dustbin-lid jellyfish” because of its large, circular shape.
Barrel jellyfish are among the largest species found in British waters, with adults capable of growing up to one metre across and weighing as much as 35kg.
They are recognisable by their translucent, rubbery domes and eight thick, frilly oral arms, which hang beneath their bodies and resemble cauliflower.
Despite their impressive size, barrel jellyfish feed on plankton and have a relatively mild sting that is generally harmless to humans.
Intrigued by the unusual discovery, Chris filmed the jellyfish and shared the footage on Facebook, where it attracted more than 60,000 views.
The video prompted a flurry of comments from people reacting to the size of the animal.
Chris said: “It was absolutely amazing, but I wouldn’t want to swim into it, I bet it looks even bigger in the water.”
However, the jellyfish did not stay on the beach for long. Chris said it appeared to have returned to the water shortly after he spotted it.
When he turned around and walked back past the same stretch of beach, the animal had disappeared.
The jellyfish is not the first unusual wildlife encounter Chris has had during his daily walks along the Bournemouth coastline.
Chris recently watched a gull battling with what he initially thought was a snake in the water, before realising it was likely to be an eel.
He said he will continue to keep an eye out for wildlife appearing across Bournemouth’s coastline.