Japan is a constitutional monarchy located on a group of islands outside the Asian mainland on the planet Earth. Its primary language is called Japanese.
Japan is also the origin of a multitude of characters including Splinter, Shredder, Hamato Yoshi, Yukio Mashimi, Karai, the Ancient One, Kenshin, Lord Norinaga, and Mitsu.
In the TMNT Adventures comics, the Teenage Mutant Ninja Turtles visit Japan, where they become friends with Ninjara.
The 1993 film Teenage Mutant Ninja Turtles III features the Ninja Turtles travelling back in time 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 Ninja Turtles and 4 other human fighters to train there and combat the reincarnation of the Tengu Shredder.
In the 2007 film prequel comic book, Leonardo visits Japan during his international pilgrimage, being tested by Hamato Yoshi's trainer.