Melmetal is a large, silver Pokémon with a vaguely 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 light gray 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 which may be ocellated. 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 nothing. According to Professor Willow, Melmetal lived in a secluded area deep within a wooded land that was full of sedimentary rocks rich with magnetite, and its shed metal was used by ancient people to create unique metal tools. 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 heavy hex nut arms. Melmetal applies centrifugal force on its arms, allowing it to have the strongest punches of all Pokémon. Melmetal is said to be created when one strong Meltan absorbs the group it lives with in order to evolve. When Melmetal reaches the end of its life, it will rust and split itself into multiple Meltan from the small shards left behind.
As Gigantamax Melmetal, it becomes a large cyclopean giant, told in legends. The dark-gray 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 a large golden nut at the center of its chest. A silver hexagonal nut is located on its abdomen, and it has a large red tail resembling an electrical wire located behind it. The head is shaped like a knight's helmet with three red clouds surrounding it. Inside is a smaller golden hexagonal nut, with a glowing red and gray sphere floating in the center of it.
In its Gigantamax state, Melmetal can use its elastic and flexible body to land punches at foes while keeping its distance. By bringing the hex nuts on its fists together, it can create gigantic fists for an even bigger punch. Gigantamax Melmetal can release tremendous electric beams from its belly, capable of vaporizing opponents and even an entire hill. The electricity is generated by the flow of liquid metal stored in Gigantamax Melmetal's body.
They are the only Pokémon in Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! whose Kanto Pokédex numbers (152 and 153) are not the same as their National Pokédex numbers (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.
They 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 Pokémon introduced in Generation VII to have a Gigantamax form.
Moreover, Melmetal is the only Pokémon whose Gigantamax form cannot be obtained through the Max Soup.
Although Melmetal's Shiny form is legitimately available, its Shiny Gigantamax form is not, as the Gigantamax Melmetal distributed in Pokémon HOME is not Shiny. Melmetal is the only Pokémon with this attribute.
While Urshifu's Shiny Gigantamax form is also not legitimately available, neither is its Shiny form overall.
Prior to the Version 1.2.0 update for Pokémon Sword and Shield, Melmetal was unable to learn Earthquake via TR, despite it being able to learn it in Pokémon: Let's Go! Pikachu and Let's Go! Eevee. This has since been fixed.
If Phione is not considered a Mythical Pokémon, Melmetal and its pre-evolution are the only Mythical Pokémon which did not debut in a movie.
Melmetal has similar design aesthetics to its pre-evolution, Meltan. Its head and features resemble hexagonal nuts, and its tail resembles an exposed wire. Its body may be based on either gallium or mercury; both are metallic elements with relatively low melting points (gallium melting at 86 °F (30 °C) and mercury being liquid at room temperature), both can conduct electricity, and both can easily react with many other metals to form alloys and amalgams. Additionally, Melmetal and its Gigantamax form may have been based on the Cyclops.