The King of Thieves is a character in Adventure Time who has only appeared in the "City of Thieves." He is the king of the City of Thieves, but he is dead. It is unknown how long it has been since the people of the city have seen him, or if he even has any authority (although, due to his name, it could be that he was the ruler of the city and he took to a life of theft, which started thievery throughout the populace). There is a treasure chest filled with gold coins and jewels in his torso, which Finn takes out. His castle is surrounded by a magical barrier that magically keeps thieves out (how he got in and out is never explained). The only entrance to his castle is a small window up high on a tall stone tower. He may be called the King of Thieves because he is very greedy and built the tower with the magical barrier to seclude his gold.
It is unknown what species he belonged to when he was alive.
There is a small game in the lower-left corner of the chest.
His eye-patch and burial surrounded by stolen treasure draws comaprisons to the pirate "One-Eyed Willy" from the film The Goonies.
The fact that the chest is in the King of Thieves ribcage is a double entendre: Penny says Finn will "find the loot in the King of Thieves chest," and the chest with the loot is in the chest of the deceased king's skeleton.