Dromaeosaurids, despite their notoriety, are poorly characterized meat-eating dinosaurs, and were previously known only from disarticulated or fragmentary specimens. Many studies have denied their close relationship to birds. Here we report the best represented and probably the earliest dromaeosaurid yet discovered, Sinornithosaurus millenii gen. et sp. nov., from Sihetun, the famous Mesozoic fish-dinosaur-bird locality in China. Sinornithosaurus not only greatly increases our knowledge of Dromaeosauridae but also provides evidence for a filamentous integument in this group. It is remarkably similar to early birds postcranially. The shoulder girdle shows that terrestrial dromaeosaurids had attained the prerequisites for powered, flapping flight, supporting the idea that bird flight originated from the ground up. The discovery of Sinornithosaurus widens the distribution of integumentary filaments among non-avian theropods. Phylogenetic analysis indicates that, among known theropods with integumentary filaments or feathers, Dromaeosauridae is the most bird-like, and is more closely related to birds than is Troodontidae.