Pokémon Tretta League Opening! (Japanese: ポケモントレッタリーグ開幕！) is the first Pokémon Tretta set. It features 48 Tretta.
Pokémon Tretta League Opening! is the inaugural Pokémon Tretta set and features a comparatively small pool of Pokémon to later sets, spread over different classes. The set predominantly includes Pokémon from Generation V, with twelve of the then-seventeen types represented. With the exception of Master-class Tretta, all are colored white, and all feature only one move. The inclusion of blank spaces on the reverse of Tretta suggested future Tretta would have multiple moves, which became the case in the following set. White Tretta have not been used in subsequent sets, with colors cycling predominantly between red, green, blue, and yellow, or shades thereof.
Pokémon can be encountered in five locations: Forest, Waterfront, Field, Ruins, and Cave. Subsequent sets feature roughly the same selection of locations with a seasonal theme, often based on when the set was released during the year.