Dialga, known as the Temporal Pokémon, is described as an ancient deity of the Sinnoh region with the ability to control time.  According to Sinnoh-region lore, it was created by Arceus, at which point time began to flow. Being able to control time, it uses its power to travel at will through the past and future. Dialga is a sauropod-like Pokémon. It is mainly dark blue with some gray metallic portions, such as its chestplate, which has a diamond in the center. It also has various light blue lines. It has a somewhat wing-like structure on its back, and a crest on its head, resembling a type of dinosaur. Dialga also has two horns on top of its head, two fang-like horns around its mouth, long claws and a short tail.

Dialga is not obtainable in Pearl, in the same way that Palkia is not obtainable in Diamond. In Diamond, it is summoned by the defeated leader of Team Galactic, Cyrus, who hopes to have it create a new universe for him, at the Spear Pillar on top of Mt. Coronet. It is stopped by Uxie, Mesprit, and Azelf, which nullify its power, and can be captured or defeated by the player. In Platinum it can be caught after finding the Adamant Orb and returning to the Spear Pillar. The Adamant Orb is used to amplify its Steel- and Dragon-type attacks when held by Dialga.

Dialga also appears in the Wii title Super Smash Bros. Brawl in the background of the Spear Pillar stage, along with Palkia and Cresselia. It will usually just stand in the background, but will occasionally attack the players and/or alter the flow of time. It appears in Pokémon Mystery Dungeon: Explorers of Time/Darkness in another form known as Primal Dialga, created when his home, Temporal Tower, is destroyed as part of Darkrai's evil plan to cause the planet's paralysis. Primal Dialga has lost almost all reason and seeks nothing but self-preservation.

Dialga appears in the tenth movie of the Pokémon anime along with Palkia. It fights with Palkia and Darkrai, nearly destroying Alamos Town. Dialga made another appearance in the eleventh movie on its return trip back to its own dimension. Dialga enters the Reverse World and is attacked by Giratina. Dialga appears once again in the twelfth movie, this time co-starring with both other members of its trio, and plays a significant role in Ash's new mission to calm Arceus after a misunderstanding related to its supposed betrayer, Damos.

In an interview, Pokémon Diamond and Pearl director Junichi Masuda noted Dialga's and Palkia's name as two of the most difficult to create, due to an effort to make them appealing to both Japanese and American audiences. During localization of the game, Spanish translators complained the name sounded too close to alga and that it gave them the impression of seaweed. Regardless, the name was left unchanged.