MissingNo. (Japanese: けつばん Ketsuban, lit. "Missing Number") is a glitch Pokémon in the original Pokémon games, known for being one of the easier and safer glitches to find and catch.

While it can appear in about four different looks, the most popular one is what appears to be a tall rectangle that's missing half of its height in about a quarter of its width in the upper left corner. The remainder of the rectangle is randomly colored pixels. The other forms it can take on are the unidentified Ghost sprite seen in the Pokémon Tower without the Silph Scope, the Kabutops fossil seen in the Pewter City Museum, and the Aerodactyl fossil also seen in the Pewter City Museum.

In Pokémon Yellow, MissingNo. takes on a more square appearance, unlike its more iconic backwards L appearance seen in Red and Blue. MissingNo. is more noticeably unstable in this version and can frequently lead to game crashes; this instability often requires player set-up to safely encounter.

MissingNo.'s appearance and stability can further vary depending on the language of the game; e.g., the Italian version of Pokémon Yellow has MissingNo. appear as a small rectangular block.