Japan

Japan is one of Earth's nation-states. Despite being wealthy and populous, it does not qualify as one of the planet's superpowers, in large part because its Constitution has severely curtailed the scope of its military since its crushing defeat by the United States of America in World War II. Its capitol is Tokyo. In some continuities it is a member state of the Earth Defense Command. This may greatly increase Japan's geopolitical clout, but it's hard to tell because the fiction is consistently very vague about how the EDC works.

It is famous for its ninjas and big, green, fire-snortin' lizards.

Generation 1

Animated Generation 1 continuity

Transformers Season 2

In 1985, Japanese scientist Dr. Fujiyama built a female ninja robot approximately the size of Optimus Prime "to benefit mankind". She didn't. Enter the Nightbird

Later that year, Japan was menaced by Kremzeek. Kremzeek

Fight! Super Robot Lifeform Transformer manga (Japan-only)

Some time between 1986 and 2002, the Autobots came from America to Japan on a longer-term basis, where they befriended an Elementary School boy named Kenji. Kenji was helpful enough to alert the Autobots about several of the numerous Decepticon attacks on Tokyo around this time, as Japan apparently became the focus of Megatron's raids.

Binaltech (Japan-only)

When Swindle released the Cosmic Rust against the Autobots in 2003, Japanese automobile companies enthusiastically participated in the EDC's plan to produce human-manufactured bodies to replace the deteriorating ones of the Autobots who were infected.

Kiss Players (Japan-only, thank God)

When Galvatron was ejected from Unicron in 2005, he impacted Earth, specifically Tokyo. The EDC's mission became ejecting all Transformers from Earth. Tokyo was quickly rebuilt, and in 2006 had a force of 48 Autroopers to defend it. Unbeknownst to the population at large, the EDC is feeding children to Galvatron's spawn.

Transformers Season 3

Some time in 2006 (2010 in the Japanese continuity), the Decepticons mounted a series of nonsensical attacks on Japan. Predaking and Devastator attacked Broadside in the Sea of Japan, upsetting local fishermen, while Astrotrain attempted to ram a bullet train, and Bruticus fought Defensor outside a dojo. Despite the able Autobot defense, the extensive damaged caused the furious Japanese Prime Minister to turn into a racial stereotype.

After questioning his fitness for leadership (and managing to lose the Autobot Matrix of Leadership in a surprise attack), Rodimus Prime learns from the dojo sensei the lesson that he who deserts his obligations is already defeated. Later, a Matrix-mutated Scourge attacked a major Japanese city. Giant robots just can't stay away from the place. The Burden Hardest to Bear

The Headmasters (Japan-only)

In 2011, the Trainbots were created and lived in Japan, disguised as different models of locomotive indigenous to the country.

Universe: The Wreckers

Some time around 2020 (Daniel Witwicky seems to be a graduate student on break from University), Japan is menaced by an entire battalion of female ninja robots approximately the size of Optimus Prime. This time Dr. Fujiyama doesn't even bother trying to explain why he had them built. Japan is saved when Daniel heroically detonates a high-powered suicide bomb, obliterating the entire group of robots.

Fun fact: this story was published in 2001.

Masterforce (Japan-only)

Japanese citizen Ginrai left his home in Nagano in his densely-populated homeland to become a trucker on the open highways of America. Meanwhile, the Pretender Hawk lived in Japan, where he was friends with Dr. Gō and his young son Shūta Gō. With the rise of a Decepticon cult made up mostly of Pretenders and humans, Ginrai found himself caught up with the Autobots and returned to Japan to stop the cult's attacks. The three children who work for the Autobots also attended a school there.

Victory (Japan-only)

A year after the Godmaster conflict (whenever that was), Star Saber enrolls his adopted human son Jean Minakaze in a school in Japan.

Marvel Comics

See Tokyo

Dreamwave

See Tokyo

Unicron Trilogy

Dreamwave Energon Comics

See sunny Tokyo one more time.

From Teletraan I: The Transformers Wiki, a Wikia wiki.