More than 300 barrel jellyfish washed up in New Quay, Ceredigion.
And in Pembrokeshire there have been sightings in Tenby, Saundersfoot and Newport.
Sarah Perry from Cardigan Bay Marine Wildlife Centre said: "This is definitely a mega swarm. I have never seen them this big before."
She said the creatures do wash up each year but added that this was "unusual because of the number on our shores and the size of them".
Ms Perry said she believed the recent warm weather had sparked the boom in jellyfish, which can grow up to 88cm (35in) in diameter.
Numbers have grown in recent years because of mild winters allowing plankton, their main food source, to thrive.
Holidaymaker Wilson Dyer, from Suffolk, said: "I've been holidaying here for 40 years and I've never seen this before.
"They're all the way up the Cardigan Bay coast. It's intriguing, it looks like they all keeled over at the same time."
Conservationist Ms Perry said the abundance of washed up jellyfish could attract feeding leatherback turtles which would be an "amazing sight".
She encouraged the public not to move or touch the jellyfish.
"While they're relatively harmless they can, if touched, leave you with a rash similar to what you may get after touching a stingy nettle," she said.
Thousands of jellyfish have washed up on beaches in Pembrokeshire and Ceredigion in what a conservationist has described as a "mega swarm".