The Fire Kingdom is state ruled by Flame Princess and home to most of the Fire Elementals of Ooo. It is entirely composed of volcanoes and seas of lava. Its monarchy reside in the Fire Palace. The Fire Kingdom made its first official appearance in "Thank You" and was a major location in "Incendium" and "Ignition Point." The majority of its inhabitants consider themselves to be evil, including the king. Fire wolf packs are scattered about the kingdom and are free to run inside the palace.
It first appears in "Evicted!" in a flashback that shows Marceline hula-hooping in a sea of flames. It is possible that the Fire Count ruled the land in the past, a flashback in "His Hero" shows him wearing a crown in what is appears to be the kingdom. The Flame King formerly ruled the Fire Kingdom, until he was deposed by his own daughter in "Earth & Water."
- Flame Princess (current ruler)
- Flame Lord
- Fire Wolves
- Lava Man
- Flame King (former ruler)
- Flame Jester (formerly)
- Flame Guards
- Flame King's brother (formerly)
- Fire Count (former, many years ago)
- Jalapeño pepper
- Cinnamon Bun
- In "My Two Favorite People," Jake, Finn, and Lady Rainicorn have a picnic on what appears to be the border between the Grass Lands and the Fire Kingdom. This is also seen in the episode's title card.
- According to one map released with the original series pitch documents, the Fire Kingdom (or "Kingdom of Fire" as it's labeled on the map) is bordered by water and the Isle of Steam to the south. A different map calls the area the "Burning Lands" and contains the warning: "Do not go here. It's on FIRE!"
- In "Evicted!," a flashback of Marceline swinging a green hula hoop around her waist was shown, and fire was in the background, meaning it took place in the Fire Kingdom. Also, Marceline said she "fooled around in the Fire Kingdom," which is more proof that Fire Kingdom was shown in "Evicted!"
- In "His Hero," Billy is shown to have saved the Cotton Candy Princess from a possible Fire Kingdom inhabitant, the Fire Count, during "Billy's Song."
- The Fire Kingdom makes a major appearance in "Thank You." It is revealed to be on the border with the Ice Kingdom, with all the ice and snow melting in the area between the two. Creatures such as Fire Wolves from the Fire Kingdom are found easily making their way into the Ice Kingdom as a result.
- In "Incendium," Jake visits the Fire kingdom in hope of winning Flame Princess to cheer Finn up. However, when Flame Princess showed her harsh and robust side, Jake immediately rejects her in fear which causes her to chase him out of the Fire Kingdom.
- In "Ignition Point," Finn and Jake revisits the Fire Kingdom to retrieve Flame Princess' scented candles.
- In "Earth & Water," Flame Princess becomes the new ruler.
- In "The Red Throne," Flame King reclaims the throne, but his second reign was short lived when Flame Princess and Cinnamon Bun successfully defeated him and Don John.
- In "The Tower," though not fully shown, Finn used some parts of it to complete his tower.
- In "Jake the Brick," it makes a cameo with Flame Princess and Cinnamon Bun in a place that seems like a library.
- In "The Cooler," Princess Bubblegum infiltrates the place by gaining the trust of the princess and weakening the citizens' life forces. It was revealed that there were several Sleeping Fire Giants, who are the ancient protectors and the greatest weapons of the kingdom, were located within the kingdom. Princess Bubblegum was able to steal several geothermic control modules but her plan was later discovered by the fire princess. The giants were eventually destroyed, with the exception of one of them by Flame Princess during her battle with Bubblegum, who managed to defeat her. Bubblegum later decided to come clean and reverted the kingdom back to its normal state by calling Ice King to stop freezing the core.
- In "The Diary," Nurse Pound Cake used the Fire Kingdom as a euphemism for hell when she said the Fire Kingdom "froze over."
- It is said that the Fire Kingdom and Ice Kingdom border each other, and although it is not visible on the map, in the episode "Thank You," one can clearly see the two bordering each other when the Snow Golem fights the wolves.
- Flame King is the arbitrary ruler of the Fire Kingdom, after snuffing his brother's flames out, which had provoked Furnius and Torcho to murder him in "Ignition Point."
- The Fire Kingdom also has a theatre, where plays are performed for both the king and the citizens' amusement, as seen in "Ignition Point."