Missingno. (Japanese: けつばん Ketsuban) is a dual-type Bird/Normal glitch Pokémon in Pokémon Red and Blue, and a dual-type Normal/999 glitch Pokémon in Pokémon Yellow. Due to the ease of which it can be found, it is one of the best-known glitch Pokémon.

In later generations, other glitch Pokémon are referred to as "a Missingno.", such as ??????????, ?, and -----. Despite this, they have little relation to the one found in Pokémon Red and Blue or Yellow.

Missingno.'s cry is the game's equivalent of a "blank" cry: a Nidoran♂'s cry with a pitch of 0 and no echo. However, a few Missingno. have different cries (specifically 43, 45, 4F, 51, 5E, 5F, 7F, and B5), such as a Rhydon's cry.

The Pokédex entries of Missingno. and all glitch Pokémon with a Pokédex number of 0 will only appear upon capture if the player has not encountered Cubone, but as with all glitch Pokédex entries, they normally remain inaccessible through the actual Pokédex. An exception to this is in Red and Green, where Missingno. can be "registered"; in the Pokédex if the player enables the 'seen' flag for #152 through select button glitches and memory corruption. If Missingno.'s Pokédex data is viewed, its cry will be heard, followed by its entry, which consists of a very long string of glitchy sounds. It is unknown why this happens.

The leveling up learnset, base stats and types of the Kabutops Fossil, Aerodactyl Fossil and Ghost forms are the same as the Pokémon whose data was accessed last.