The Enantiornithes, or "opposite birds", are an extinct group of flying birds. Early discoveries were incorrectly referred to modern bird groups. They were first recognized as a distinct "subclass" by C.A. Walker in 1981, based on some partial remains from the Late Cretaceous of Argentina. It the 1990s, more complete enantiornithines were discovered, and it was more demonstrated that a few previously found birds (Iberomesornis, Sinornis) have enantiornithine features.

Enantiornithines have been found in North America, South America, Europe, Asia, and Australia. Known fossils attributable to this group are exclusively Cretaceous, and it is belived that enantiornithines became extinct at the same time as their non-avian dinosaur relatives. One biogeobraphic study in the 1990s suggested that the distribution of enantiornithines implies a Middle Jurassic origin for the clade, but this theory has not been widely accepted by paleoornithologists. The earliest known enatiornithines are from the Early Cretaceous) of Spain (eg. Noguerornis) and China (eg. Eoenantiornis), and the latest from the Late Cretaceous of North and South America (eg. Avisaurus).

Enantiornithines were more advanced than Archaeopteryx, but more primitive than all living birds (Neornithes). Over 30 species have been named, but probably not all are valid. All but the most primitive enantiornithes belong to a clade called Euenantiornithes. Some had many teeth, but others were toothless. They have been found in both inland and marine sediments, suggesting that they were an ecologically diverse group. The smallest enantiornithines are described as sparrow-sized, but Enantiornis was much larger, with an estimated wingspan of 1.2 m.

Although many frustratingly incomplete specimens of enatiornithines have held back attemts to understand the phylogeny of this group, some examples are spectacularly well-preserved with even their feathers in place. A few examples of embryonic enantiornithines have been found inside fossil eggs. Recently, palaeontologists in China found a enantiornithine fossil with flight feathers on its legs as well as its arms, linking it with the four-winged dinosaur Microraptor. This may suggest that some early birds had four wings, not two. However, the leg feathers of the enantiornithine differ from those of Microraptor in being shorter, and only extending down to the ankle rather than along the foot as in the four-winged dinosaur.