Pokemon.com announced the title of the twelfth Pokémon movie today, Pokémon: Arceus and the Jewel of Life. Originally announced during the Locarno International Film Festival in August, the title was speculated to be the official title. Below is the press release.

A Tale Untold. A Legend Unleashed.

Get ready for the latest Pokémon movie, Pokémon: Arceus and the Jewel of Life! This final installment in the three story arc reveals that the Legendary Arceus is at the center of the recent disruptions in the Pokémon World. First the epic battle that threatened to tear a hole in the space-time continuum took place between Dialga and Palkia. Then Giratina, the sole defender of the Reverse World, had to take action to protect the precarious balance of its home and the Pokémon World. Now, Michina Town is at the heart of the action. The town has a long history and checkered past in relation to the mighty Arceus - after Arceus granted the townspeople some of its power as the Jewel of Life, they betrayed the Pokémon!

Ash and his friends, accompanied by their new companion Sheena, travel in time with Dialga's help to right the mistakes of the town's ancestors before the world crumbles under the power of Arceus's rage. Will they be able to fix the mistakes of the past? What really happened in the past - did Sheena's ancestors truly betray Arceus? What the stunning finale to the trilogy when all of the secrets of the previous two movies will be revealed!

Source: http://www.pokemon.com/#news_/XML/news_402.xml