The Master Ball is the rare Poké Ball that never fails when used to catch a Pokémon. In the main Pokémon games, throwing one at an eligible wild Pokémon guarantees the capture, regardless of its remaining HP, level, or ordinary catch rate.
The Master Ball was introduced in Pokémon Red, Blue, and Yellow as an extremely valuable item. In those games, players receive one from the president of the Silph Company after Team Rocket is defeated at Silph Co. Because most games provide only one through the main story, trainers often save it for especially difficult encounters.
Legendary Pokémon are the classic targets, but using a Master Ball is not always the best strategic choice. Some rare Pokémon can be caught with ordinary balls after weakening them and applying status conditions. Players may also reserve the Master Ball for roaming Pokémon that flee from battle or for a particularly difficult shiny Pokémon.
Its guarantee applies within the game’s normal catching system. It cannot override every special situation: some encounters cannot be caught, and the ball may be unavailable or unusable in certain battles or gameplay modes.