12 May 2014 Last updated at 15:29 BST
It's thought that the plankton they eat is in bloom close to shore.
Strong winds brought the tide high and left some of the strange creatures stranded.
This type of jellyfish is the biggest found in British waters.
Although they are not dangerous, people are being advised not to touch them.
Hear more about the jellyfish from wildlife photographer Steve Trewhella.
Dozens of huge barrel jellyfish have been found washed up on beaches in Dorset.