Greninja is a large, bipedal, frog-like Pokémon. Its body and legs are dark blue, with single large white bubbles on its legs and arms and white spots over its eyes. On the sides of each of its upper thighs is a light blue, four-pointed star, which resembles a shuriken. Its back feet have two toes, while its front feet have three toes. Its feet are webbed, and it has a yellow chest and a yellow-and-blue face. It has eyes with white sclera, red irises, and white pupils. A large, projecting dark blue stripe runs down the middle of its head, with projecting light blue stripes in between its large, yellow ears. Its mouth is hidden behind a large, pink tongue that wraps around Greninja's neck and extends outward behind its head.
Greninja moves with the speed and grace of a ninja, using swift movements to confuse its enemies while it slices them up with throwing stars made of compressed water; these throwing stars are sharp enough to split metal.
Through a transformation called Bond Phenomenon, Ash's Greninja in the anime and any Greninja with the Ability Battle Bond in Generation VII can change into an alternate form called Ash-Greninja. In this form, Greninja's skin turns a lighter shade of blue, while its face gains red stripes and its cheeks and ears turn black, changing shape to resemble Ash's hair. Its legs are stronger in this form, and it gains the ability to form large Water Shurikens on its back.
Greninja is based on a frog, and may have been inspired by horned frogs from the generaMegophrys and Ceratophrys due to the spiked protrusions above its eyes. Its coloration and body shape is similar to the splendid leaf frog. The bubble-like protrusions on its body may be based on a toad's parotoid glands, possibly referencing the traditional myth of toads causing warts, though they may also be remnants of Froakie and Frogadier's frubbles. As its species name implies, Greninja is also based on a ninja. Japanese culture associates frogs and toads with ninjas based on the folk story "The Tale of the Gallant Jiraiya" (児雷也豪傑物語 Jiraiya Gōketsu Monogatari) about a ninja who can shapeshift into a large toad. Greninja and its pre-evolutions may also draw inspiration from the thief or rogue class as they often rely on speed and stealth and are often seen as nimble or ranged combatants while tending to focus more on dodging attacks rather than withstanding damage.
Ash-Greninja's appearance is based on Ash's attire in the XY series.
Greninja is a combination of grenouille (French for frog) and ninja.
Gekkouga is a combination of げこっ geko (croak) and 甲賀流 Kōga-ryū. It may also derive from 月光 gekkō (moonlight), in reference to a popular conception of ninja running through the night or to Greninja's Dark typing, or a combination of both.