Seedless watermelon is a

Seedless watermelon is a Triploid.
Seedless watermelons are triploid (3X) which causes them to be sterile, or seedless. The triploid seeds are created by crossing a normal diploid (2X) melon as the pollinator with a tetraploid (4X) parent. Each parent contributes half its respective chromosomes, 1X from one parent and 2X from the other.
It is a sterile hybrid that is created by crossing male pollen for a watermelon, containing 22 chromosomes per cell, with a female watermelon flower with 44 chromosomes per cell.