Jirachi is a small, white, humanoid Pokémon. It has short, stubby legs and comparatively longer arms. There are flaps on the underside of its arms, which give the impression of long sleeves. On its belly is a curved seam, which conceals a third, yellow eye with a cyan pupil, known as its "true eye". Additionally, Jirachi has a normal pair of ovalish eyes on its face, with small, blue triangular markings underneath. On its head is a large, yellow structure with three points extending outward: one from the top and one on either side. On each point is a teal tag, known as a "wish tag". A rounded extension hangs from either side of the yellow structure, framing Jirachi's face. Two yellow streamers flow from Jirachi's back, resembling comet tails.

Jirachi hibernates for most of its life, awakening for seven days every thousand years. It can be also awakened if sung to by a voice of purity. Jirachi sleeps wrapped up in its yellow streams similar to a baby. While it sleeps, a tough, crystalline shell envelops its body to protect itself. However, even without, Jirachi can still fight while asleep. During the short periods it is awake in, Jirachi will grant wishes. However, how its wish powers work differ between canons. In the Jirachi: Wish Maker movie, Jirachi cannot create objects through its wishes, instead, it'll teleport the wished object to the one who wished for it. By contrast, in the Pokémon the Series: Black & White episode Searching for a Wish!, it can grant wishes without the use of teleportation. In the Pokémon Adventures manga, Jirachi will only grant wishes when its true eye is opened. Additionally, it can only grant three wishes every thousand years, which are written on each of its three wish tags. It was also shown being able to create artificial Pokémon through its wishes, animating a mass of seawater in the shape of a Kyogre. Jirachi's power is similarly exploited in the movie to create a monstrous Groudon. The movie further reveals that Jirachi's awakening is linked to the appearance of the Millennium Comet and that its third eye has the ability to absorb the energy Jirachi needs for hibernation. In Pokémon Adventures, Jirachi was shown to slumber in space. It is said that Jirachi's ability to grant wishes also gives it great attack power, as seen when it creates a concentrated bundle of light to blast its target. Although it is extremely rare, Jirachi can be found in the mountains.