A gas giant is a huge planet made entirely of base gases, including hydrogen, helium, methane and ammonia. Typically they do not posses a clearly distinguishable solid surface, but rather have dense atmospheres. These turbulent atmospheres are generally made of the same materials as stars, however, they lack the mass necessary to begin nuclear fusion.

There are several types of gas giants known in Star Trek: Class J, Class 6, Class 7, and the Class 9 gas giant, which is also known to the Klingons as the Q'tahL class. (DS9: "Starship Down"; ENT: "Broken Bow", "Sleeping Dogs") The Sol system has four gas giants, none of which are Class 9. (ENT: "Sleeping Dogs"). In 2152, Enterprise pulled the IKS Somraw from a Class J gas giant. (ENT: "Sleeping Dogs", "Marauders", "Judgment") Later that year, they encountered a gas giant in Arkonian space that had "dozens of moons," and was referred to as "like a gravitational jigsaw puzzle." (ENT: "Dawn") n the mirror universe, the Tholians held the USS Defiant at a facility in the Vintaak system that was orbiting the gas giant. (ENT: "In a Mirror, Darkly")

In 2369, the USS Enterprise-D was able to witness the collision of two gas giants in the Detrian system. (TNG: "Ship in a Bottle"). In 2370, the USS Raman became trapped in the atmosphere of a gas giant while the crew was attempting to sample it at a low orbit. (TNG: "Interface").

In 2371, a Maquis interceptor, piloted by the Female Changeling, lured a runabout, piloted by Kira Nerys and Odo, to a moon orbiting the gas giant located near the Badlands. (DS9: "Heart of Stone").

Astronomically, a gas giant (sometimes also known as a Jovian planet after the planet Jupiter) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars.

Gas giants can be subdivided into different types. The "traditional" gas giants, Jupiter and Saturn, are composed primarily of hydrogen and helium. Uranus and Neptune are sometimes considered a separate subclass called ice giants, as they are mostly composed of water, ammonia, and methane; the hydrogen and helium in Uranus and Neptune is mostly in the outermost region. Among extrasolar planets, Hot Jupiters are gas giants that orbit very close to their stars and thus have a very high surface temperature; due to the relative ease of detecting them, Hot Jupiters are currently the most common form of extrasolar planet known.