Like Mewtwo, Mew is a Psychic-type Pokémon with high stats. Morimoto designed it as a pink, feline-esque Pokémon with large eyes and a long, thin tail that broadens at the end. Its skin is covered with a layer of short, fine hair. Its DNA combines the genetic composition of all existing Pokémon species;^[11] the game states that scientists within the game view it as being the single ancestor of all other Pokémon. It is shy and rarely seen by humans. It is a legendary Pokémon from the first generation, along with Articuno, Zapdos, Moltres, and Mewtwo. Mew's number in the National Pokédex is 151, the last of the first-generation Pokémon, with 150 being Mewtwo and 152 being Chikorita. In the first generation games and their remakes, the player can find journal entries in the Pokémon Mansion on Cinnabar Island stating that Mew was discovered deep in the jungles of Guyana, South America, on July 5 (the year is not specified), and named on July 10,and that it "gave birth" to Mewtwo on February 6. The name Mew is based on the onomatopoeia of a cat's cry, meow. Mew is extremely adorable and can transform into any Pokémon at will. It is the strongest Pokémon, including its clone, Mewtwo. Mew is a female, based off a jerboa. Mew's name was originally going to be Jermew, but soon was changed to Mew.

In the video games, it is possible for Mew to learn any move that can be taught.Other than Ditto, it is the only Pokémon that can transform into other Pokémon using the "Transform" technique. In the anime, it is capable of flight, teleportation, shapeshifting, summoning giant pink bubbles of psychic energy (which serve various purposes such as closing itself in for protection, acting as a cushion, or simply for Mew's amusement of bouncing on), and rendering itself invisible.