"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 Aptian age of the Early Cretaceous period, which makes it among the earliest and most primitive dromaeosaurids yet discovered. Fossils of "S. haoiana" and the possible "Sinornithosaurus" specimen NGMC 91 were found in younger strata dating to about 122 million years ago. The presence of vaned feathers on "Sinornithosaurus" is consistent with feather evidence from other dromaeosaurs.