Which Pokémon film was the first in the series to premiere in an IMAX theater?

Pokémon 3: The Movie was the first Pokémon film to premiere in an IMAX theater.

The film, also known as Pokémon 3: The Movie—Spell of the Unown, is the third animated Pokémon feature. Its story follows Ash, Misty, and Brock as they encounter the Unown and try to help Molly Hale, whose loneliness leads to an extraordinary transformation of her surroundings.

The IMAX presentation was especially notable because the Unown's crystallization effects were designed to create a striking three-dimensional visual experience. This makes the film a distinct milestone in the franchise's theatrical history, rather than merely the third entry in release order.

The movie first reached Japanese theaters on July 8, 2000, and later opened in North America on April 6, 2001. It is often confused with Pokémon: The First Movie or Pokémon the Movie 2000, but neither of those earlier films was the series' first IMAX premiere.