A highly advanced sentient — apparently crystalline species, non-humanoid in appearance and extremely territorial in disposition. They hail from a hot planet by human standards, probably not Class-M. Together, two Tholian ships can literally spin a web of energy in space in order to trap an enemy vessel, after which the web is then drawn together, constricting and finally destroying the vessel inside. On stardate 5693.2, the U.S.S. Enterprise NCC-1701 made first contact with this species.

Tholians are a hive-mind and known for their punctuality. One of the independent border races that could have been dragged into a Klingon civil war, due to their proximity.

During a period of renewed conflict with the United Federation of Planets (UFP) in 2253, they attacked a starbase and killed the entire complement — except civilian advisor Kyle Riker, who refused to give up survival. In a battle simulation during his Academy days, Riker figured out the sensory blind spot of a Tholian ship and hid there.

Tensions have eased enough for the Tholians to send an ambassador to space station Deep Space Nine.

In a Mirror reality, circa 2158, the Tholians secretly experiment on the U.S.S. Defiant, a starship snared in a Tholian web and somehow brought to this alternatie universe, 100 years before it was taken. The existence of the future starship in Tholian space of the past contributes to the theory that the Tholians somehow transcend normal space by operating and navigating some sort of "interphasic rift."