The Crystal Sword is a weapon seen used by Fionna. It was given to her by the Ice Queen disguised as Prince Gumball. The sword has the appearance of a ruby, and it has a pinkish glow. The sword is also retractable, akin to a lightsaber. At the Ice Queen's command though, the sword turns into a giant block of ice, encasing the wielder's hands. Fionna proved that this too was an adequate weapon by using the ice block to beat Ice Queen over the head and shatter the icicle encasing Prince Gumball.
- Fionna calls it a Crystal Sword when she first received it, however she later calls it a retractable sword. The name change may be because it can retract to just a crystal making it easy to travel with.
- The Ice Sword is its true name, as said by Ice Queen when she exclaims, "HAHAHA! You like my Ice Sword?"