Sunfish (or Mola mola) are the world's heaviest bony fish. They can grow to be 3.3m long! The heaviest recorded sunfish was 2,300kg - that's about the size of a white rhino!
Sunfish are famous for their strange shape. Some people say they look like a massive fish head with a tail.
Not much is known about sunfish but new technology is helping scientists from the University of New Hampshire in America find out more about them.
They've been tagging sunfish with satellite tags to find out where they are and what they are doing. They've found that sunfish can dive to great depths and travel big distances.
Some of the tagged sunfish swam from the northeast American coast to the Caribbean - that's a distance of more than 1,000 miles!