A mysterious group of related species located in the Delphic Expanse, all of whom evolved to spacefaring technological intelligence at the same time on the same homeworld, Xindus.

In 2153, the Xindi attacked Earth with a deadly probe, cutting a swath from Florida to Venezuela and killing 7 million — all because of false information fed them by the Guardians, one faction in the Temporal Cold War. The small-scale test was to be followed by a larger weapon to destroy the planet entirely — all to pre-empt what the Xindi are told will be an Earth attack on their own planet in 400 years. That "news" is in fact, however, a lie by the Guardians to further their position in the war, and is finally realized by a majority of the Xindi in time to barely avoid the attack.

There are five distinct species of Xindi: Xindi-Primate (initially called "Xindi-Humanoid"), Xindi-Reptilian, Xindi-Arboreal (sometimes called "Xindi-Sloth"), Xindi-Aquatic and Xindi-Insectoid. A sixth species, the Xindi-Avians, are now extinct: they were killed off in a war that went on for nearly a hundred years, destroying Xindus and spreading the other species to nearby habitable planets. In the mid-22nd century the Xindi Council was formed to reunify the surviving races, but their attention was diverted from a peaceful agenda to an aggressive one in order to address the alleged "human threat." Despite their varied forms and typical mistrust, they all share a common physicality: a short vent-like opening in the "cheek."