The only known specimen was discovered in Xixia County, Henan Province, in central China, and became the holotype of the new genus and species Xixiasaurus henanensis in 2010.

The specimen consists of an almost complete skull (except for the hindmost portion), part of the lower jaw, and teeth, as well as a partial right forelimb.

Unlike in most troodontids, the teeth of Xixiasaurus did not have serrations; instead, their carinae (front and back edges) were smooth and sharp.

[1] In 2014, the palaeontologist Takanobu Tsuihiji and colleagues stated that a bone Lü and colleagues had originally identified as the vomer (part of the palate) of Xixiasaurus was instead part of the premaxillae or maxillae (the main bones of the upper jaw), based on comparison with the vomer of the more complete troodontid Gobivenator.

As a troodontid maniraptoran, it would have been bird-like, lightly built, with raptorial (grasping) hands and an enlarged sickle-shaped claw on the hyper-extendible second toe.

The nasal process of the premaxilla ended at the level of the hind margin of the external narial opening (bony nostril), and was quadrangular in cross-section rather than triangular, as in Byronosaurus.

[1] The maxilla was much longer than high, forming most of the skull's side surface, and to the front, most of the lower margin of the narial opening.

The narial opening was roughly oval, with its long axis (the longest stretch from its front to the back) being parallel with the lower margin of the maxilla.

On the lower side, the maxilla formed an extensive internal shelf which contributed to a large secondary palate (which separated the nasal airways from the mouth), extending backwards from the contact with the premaxilla.

[1] It differed from other troodontids in that the front end of the dentary of the lower jaw was downturned (also called "a chin-like eminence"[7]), similar to what is seen in therizinosaurian theropods.

The mandibular symphysis (the area where the two halves of the mandible connected at the front) was short, and this region was slightly curved towards the middle.

A narrow, deep Meckelian groove ran at the inner side of the dentary, just above its lower margin, towards the front of the mandibular symphysis.

[1][7] Unlike most troodontids, Xixiasaurus did not have serrations on its teeth, and their carinae (front and back edges) were instead smooth and sharp, as in Byronosaurus.

They found Xixiasaurus to be most closely related to Byronosaurus of Mongolia, and suggested the two may have formed a clade with Urbacodon from Uzbekistan consisting of troodontids with unserrated teeth, which radiated across Asia (while noting that serrations had been lost independently in different groups of theropods).

[1] A 2012 phylogenetic analysis by the paleontologist Alan H. Turner and colleagues instead found Xixiasaurus to belong in a clade with Sinovenator and Mei (both also from China), due to sharing a maxillary process of the premaxilla that separated the maxilla from the nasal behind the narial opening.

Jinfengopteryx elegans Mei long Sinovenator changii Daliansaurus liaoningensis Sinusonasus magnodens Sinornithoides youngi Troodon formosus Zanabazar junior Saurornithoides mongoliensis In 2019, the palaeontologist Scott Hartman and colleagues recovered Xixiasaurus as the sister taxon of Sinusonasus, in a clade with Daliansaurus and Hesperornithoides (sharing features such as a straight ulna and having an upwards projected curve on the claw of the first finger).

[10] A 2021 study by the palaeontologists Andrea Cau and Daniel Madzia found Xixiasaurus to group with Zanabazar and Saurornithoides.

Troodontids probably originated in Asia, and if genera such as Anchiornis are considered part of the family, it would have evolved by the Early Jurassic.

[12][1] Due to their large brains, possible stereoscopic vision, grasping hands, and enlarged sickle-claws, troodontids were generally assumed to have been predatory.

They suggested that this difference in coarseness may be related to the size and resistance of plant and meat fibres, and that troodontids may have been herbivorous or omnivorous.

They also pointed out that some features that had been interpreted as predatory adaptations in troodontids were also found in herbivorous and omnivorous animals, such as primates and raccoons.

They stated that troodontid anatomy was consistent with a carnivorous lifestyle, and pointed out that the structure of their serrations was not much different from those of other theropods.

[16] The formations of the Xixia Basin are thought to be of continental origin (due to the absence of marine fossils), probably deposited by rivers and within lakes.

[15] The palaeoclimate of the Xixia basin is inferred to have been subtropical, sub-humid to sub-arid, based on the preponderance of C3 plants identified in the diet of the dinosaurs there (determined through isotopic study of egg shells).

[18][19] The discovery of abundant and diverse dinosaur eggs in Henan Province has been considered "one of the significant scientific events" in China.

Dinosaur ootaxa (taxa based on eggs) from the Majiacun Formation include Prismatoolithus (which may belong to troodontids), Ovaloolithus, Paraspheroolithus, Placoolithus, Dendroolithus, Youngoolithus, and Nanhiungoolithus.