Tholians are a starfaring and extremely territorial race in the Star Trek fictional universe. The Tholian home nation is the Tholian Assembly, which inhabits an area of space in the Alpha Quadrant, in close proximity to the Cardassian Union and the Badlands. The Tholian Assembly is located rimward of the Cardassian Union and the Breen Confederacy, and the homeworld of Tholia is a Class Y planet.

Non-humanoid in form, Tholians have crystalline bodies, two arms, and six legs. They dwell in an environment of extreme by human standards, their comfortable living environment being 480 kelvins (207 °C). Temperatures below 380 K (107 °C) will kill Tholians, causing them to shatter into tiny crystalline shards. Each Tholian has both male and female sexual organs. Natural Tholian speech is a series of shrill squeals and clicks, similar to the sounds of a dolphin. Their speech is difficult to parse through the universal translator. Tholians have little tolerance for deception, and are insistent on punctuality. Tholian silk is highly prized and difficult to obtain. It is an urban legend that Tholian silk is actually made from Tholians, not by them, although there is no actual evidence in the Star Trek canon.

Besides utilizing standard offensive weaponry, Tholian ships can also work in concert to emit an energy field shaped much like a web, which will trap any space vessel inside and drain its energy. When more ships are available, a web takes much less time to construct; several Tholian vessels working in concert can spin such a web in under a minute (as seen in "In a Mirror, Darkly"), while two vessels might take hours ("The Tholian Web"). The Tholians appear to actively explore the science of inter-spatial rifts and time travel. Tholians from both the regular Trek universe and the Mirror universe are depicted as engaging in missions to capture future technology, ostensibly to use the advanced technology to further their own agenda.