Melmetal has a Gigantamax form, which is currently unobtainable.
It was officially revealed on October 24, 2018.
Melmetal is a large, silvery Pokémon with a humanoid build that consists mostly of liquid metal. Many dark gray hexagonal nuts are embedded in or attached to its body: one around each shoulder, one around the center of each arm, one in each hand, one in each foot, and a small one in the center of its torso. A large, golden hexagonal nut encircles its neck and a smaller one acts as its head. Floating in its head is a small, dark gray sphere. It has a small red tail resembling an electrical wire.
Melmetal is a Pokémon that has come back to life after 3,000 years. In ancient times, it was worshiped for its ability to create metal from anything, which was used to create unique metal tools. According to Professor Willow, Melmetal was considered a sacred being that lived in a secluded area deep within a wooded land that was full of sedimentary rocks rich with magnetite. When Melmetal reaches the end of its life, it will rust and split itself into multiple Meltan from the small shards left behind. The process of evolution has hardened its body, making it incredibly sturdy. However, it is still in a liquid state and has some flexibility, particularly in its arms. The strength of the heavy hex nut arms comes from the centrifugal force, which allows Melmetal to have the strongest punch of all Pokémon. Melmetal is said to be created when one strong Meltan absorbs the group it lives with in order to evolve.
As Gigantamax Melmetal, it becomes a large cyclopean giant. The dark grey hexagonal nuts are now embedded as five fingers for its hands, two for the feet, two underneath the shoulder, and two connecting the shoulder to the large gold, golden, decagonal nut. A silver hexagonal nut is located between Melmetal's legs and chest, which has a large red tail resembling an electrical wire located behind it. The head is shaped like a knight's helmet with a smaller golden hexagonal nut located inside, with a glowing red grey sphere floating inside the head. Three red clouds surround the head. Gigantamax Melmetal can release tremendous electric beams from its belly, capable of vaporizing the opponent in one shot.
They are the only Pokémon in Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! whose Kanto Pokédex number (152 and 153) is not the same as its National Pokédex number (808 and 809). While Kanto Pokédex numbers are normally used throughout the game, the judge screen displays National Pokédex numbers, which is evident for Meltan and Melmetal.
They are the only Pokémon obtainable in Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! that cannot have the Let's Go origin mark.
Meltan and Melmetal are the only Pokémon in the National Pokédex of Pokémon GO and Pokémon HOME (Switch only) to not be listed in a region section. Instead, they are put into "Unknown" and "Unknown Origins" respectively.
Melmetal is also the only Generation VII Pokémon to have a Gigantamax form.
Melmetal is coded to learn Thunder Punch upon evolving. However, this is not possible due to Meltan being unable to evolve in the core series games.
Melmetal has similar design aesthetics to its pre-evolution, Meltan. Its head and features are based on hexagonal nuts, and its tail resembles an exposed wire. Its body is based on mercury, a metallic element that is liquid at room temperature. Mercury can also conduct electricity, as well as react with many other metals to form amalgams.