Which Pokémon generation introduced the Sinnoh region in Pokémon Diamond and Pearl?

Generation IV introduced the Sinnoh region in Pokémon Diamond and Pearl.

Developed by Game Freak for the Nintendo DS, Diamond and Pearl first released in Japan on September 28, 2006. They were the fourth generation of the core Pokémon series and used the Nintendo DS’s touchscreen and wireless capabilities as part of the games’ design.

Sinnoh is a mountainous region with locations inspired by Japan’s Hokkaido and surrounding areas. Its legendary Pokémon are central to the paired versions: Dialga is associated with Diamond, while Palkia is associated with Pearl. The expanded Pokédex also added new evolutions for several Pokémon from earlier generations.

Diamond and Pearl are sometimes remembered mainly for their 3D-looking environments, but they were not fully 3D games in the modern sense. Much of the presentation combines 2D character and environment elements with three-dimensional effects. Their online and Wi-Fi features nevertheless marked a major step for worldwide Pokémon trading and battling.