The X-Nauts (pronounced "Cross-Nauts") compose a vile group led by the malicious Sir Grodus and his servant Lord Crump, and are the weakest forms of the X-Naut army, according to Goombella, in the fictional Mario universe. They are short and fat, wear a red uniform with a black X, and have two goggles sticking out under a large bandana-like piece of cloth. Two small floppy ears poke out on the sides. Saluting an officer is done by crossing their arms across their chest. The X-Nauts' focus is to open the title door in Paper Mario: The Thousand-Year Door. They work in a secret base on the moon, and may be extraterrestrial.

As a rule, they are dim-witted and easily fooled, as well as being individually weak, but will rush to the call of their master and defend him. The wit of Sir Grodus's second-in-command and pawn Lord Crump is probably the dimmest. He likes working with machines and his machine-monster Magnus von Grapple.

Like most Mario villains, the X-Nauts threaten the Mushroom Kingdom. As has occurred in past role-playing games in the Mario universe (Super Mario RPG and Mario & Luigi: Superstar Saga), they are an original enemy to go with the plot of the game, when the normal Bowser threat is deemed too predictable.