Obsidius (オブシディウス Obushidiusu) is a volcanic kaiju that first appeared in the 2007 Godzilla video game Godzilla: Unleashed. He was chosen for the game through an IGN readers' poll, defeating The Visitor, Fire Lion, and Lightning Bug with 37% of the votes.

Obsidius' name comes from "obsidian," a type of volcanic glass. It was chosen by a second IGN readers' poll after he was voted into the game, prevailing over Pyrodorah, Kazango, Dotoryo, and Volcanis. During the original poll, he was called Magmouth.

Originally, according to art director Robert Caracol, Obsidius "was a combination of two separate concepts. The first was a rocky, craggy creature that was solid rock. The second was more akin to flowing magma. In the end, we came up with a blending of the two ideas, arriving at a monster that was a walking volcano. It possessed the hardness and sharpness of solid granite but could also attack using the molten fluid from within its body." Obsidius' design did not change much from there. Its colors range from a dark brown to charcoal black, and then its magma ranges from yellow to orange. It has three horns at the back of its head, no eyes, no mouth features—other than a hole similar to a throat where magma can be expelled from—and no real features other than rocky skin and thin lines of magma where articulation is required, like the joints of the arms, legs, neck, tail, and feet.

Obsidius can spit out torrents of molten lava and condense it into a beam that is capable of melting buildings with ease and burning other opponents.

Rip it up: Obsidius performs this close-ranged attack by digging his hands into the Earth and then tearing the ground upwards, striking the body of his opponent with his clawed fingers while throwing chunks of rock and dirt into the opponent kaiju's face.

Obsidius is an exceptionally strong brute with powerful hand-to-hand fighting capabilities. Many of Obsidius' attacks are similar to a gorilla and Orga, with his huge arms and flexible legs playing a huge role in his fighting style. While being a bit slower than fellow mutant Krystalak, Obsidius can curl himself into a ball and roll around at high speeds. He also appears to be an extraordinary jumper, able to leap to incredible heights, jumping hundreds of meters in the air.

Being made of the raw, ferrous materials of the Earth, Obsidius dons a virtually impenetrable mineral exterior that shields him against both physical and energy assaults. He is also immune to lava, allowing him to roam around in the pyroclastic lava pools in the Seattle arena of the game.

A strange ability that Obsidius possesses, the volcanic-rock kaiju is capable of absorbing certain properties of his opponents when grappling them, depending on if they are living or non-living. If the foe he grabs is organic, Obsidius steals some of their life force and heals himself; if mechanical, Obsidius saps some of their energy to replenish his stamina bar. This ability is exclusive in the PlayStation 2 version of the game.

As shown in his animatic, Obsidius can burrow underground. This ability was not demonstrated in-game.