Japan is a monarchy located on a group of islands outside the Asian mainland on the planet Earth. It is also the origin of a multitude of characters including Splinter, Shredder, Hamato Yoshi, Yukio Mashimi, Karai, and the Ancient One.
The Ninja Tribunal and their Acolytes friends travel to Japan to train in the 2003 series.
In the Teenage Mutant Ninja Turtles Adventures comics, the Teenage Mutant Ninja Turtles visit Japan, where they become friends with Ninjara.
The 1993 film Teenage Mutant Ninja Turtles III features time travel to feudal Japan. The 1993 The Legend of Koji episode of the 1987 Teenage Mutant Ninja Turtles TV series also features time travel to feudal Japan.
In the 2003 animated series, Japan played a more important role during the series's fifth season. It was the home of the Ninja Tribunal, where they kept a monastery called The Lap of the Gods. They had recruited the Turtles and 4 other human fighters to train there and combat the reincarnation of the Tengu Shredder.