Ultra Forest (Japanese: ウルトラフォレストUltra Forest) is the home of Kartana, located several thousand light-years from Alola. It is accessible primarily through Rarity 3 warp holes in Ultra Warp Ride, rarely through Rarity 2 warp holes and very rarely through Rarity 1 warp holes. It only appears in Pokémon Ultra Sun.
The landscape of this world is that of a forest, with trees sprouting flowers resembling the world's Ultra Beast, Kartana. The leaves on the floor are rectangular and resemble paper. The ground appears to be sandy, resembling a Japanese rock garden. At the end of the area is a large rock where Kartana can be fought. On the first encounter, Kartana slashes the rock in two before battle.
The speech of the Trainers here are a reference to kendo, a Japanese martial art.
Due to having to battle the Ultra Forest Kartenvoys first before encountering Kartana for the first time, when the player fights Kartana for the first time, its actual name will be displayed rather than the usual "???" (due to the fact that Kartana has already been encountered during the battle against the Ultra Forest Kartenvoys).