The setting region in Pokémon Diamond and Pearl is Sinnoh.
Sinnoh is the fictional region where the fourth-generation games take place. It is a mountainous, northern-looking land inspired by Japan's Hokkaido, with Mt. Coronet forming a major dividing range through the region. The games' story sends the player across Sinnoh's towns, routes, lakes, and Pokémon Gyms.
The region is also defined by three major lakes—Verity, Acuity, and Valor—which are connected to the mythology of the legendary Pokémon Uxie, Mesprit, and Azelf. Team Galactic exploits that mythology in an attempt to reshape the universe using Dialga or Palkia.
Sinnoh is sometimes confused with Hisui, the older version of the same land shown in Pokémon Legends: Arceus. It is also distinct from Hoenn, the setting of Ruby and Sapphire, and Kalos, the setting of X and Y. Beneath Sinnoh lies the Underground, a major exploration and multiplayer area introduced in Diamond and Pearl.