Sinornithosaurus (derived from a combination of Latin and Greek, meaning 'Chinese bird-lizard') is a genus of feathered dromaeosaurid dinosaur from the early Cretaceous Period (late Barremian) of the Yixian Formation in what is now China.

The second type, including those on the arms, were composed of rows of filaments (barbs) joined along a main shaft (rachis), making them similar in structure to modern bird feathers.

[7] A 2010 study indicated that Sinornithosaurus may have had feathers which varied in color significantly across different regions of the body, based on analysis of microscopic cell structures in preserved fossils.

[9] In 2009, a team of scientists led by Empu Gong examined a well-preserved Sinornithosaurus skull, and noted several features suggesting it was the first-identified venomous dinosaur.

Gong and colleagues noted that the unusually long and fang-like mid-jaw (maxillary) teeth had prominent grooves running down the outer surface, towards the rear of the tooth, a feature seen only in venomous animals.

They also demonstrated that the teeth were not abnormally long as Gong and his team claimed, but rather had come out of their sockets, a preservational artifact common in crushed and flattened fossils.

Finally, they could not independently verify the presence of supposed chambers for venom glands cited by Gong's team, finding only the normal sinuses of the skull.

They admitted that grooved teeth were common among theropods (though they suggested they were really only prevalent among feathered maniraptorans), and hypothesized that venom may have been a primitive trait for all archosaurs if not all reptiles, which was retained in certain lineages.

Sinornithosaurus was a member of the family Dromaeosauridae, a group of agile, predatory dinosaurs with a distinctive sickle-shaped toe claw, which also includes Deinonychus and Utahraptor.

S. millenii lived about 125 million years ago in the Barremian age of the Early Cretaceous period, which makes it among the earliest and most primitive dromaeosaurids yet discovered.

A second species, S. haoiana ("Hao's Chinese bird-lizard") was described by Liu et al. in 2004 based on a new specimen, D2140, which differed from S. millenii in features of the skull and hips.

[20] Ji, along with another team of scientists, further emphasized this similarity in a 2002 paper, in which they formally referred the specimen to Sinornithosaurus, though they considered the exact species questionable.

As the two specimens were "identical in all character states" used in his phylogenetic analysis, were from similar stratigraphic levels and "uniquely share the presence of a triangular coracoid with a large, oval foramen", Senter concluded that NGMC 91 does belong to the species Sinornithosaurus millenii.