Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations. The first seabirds evolved in the Cretaceous period, while modern seabird families emerged in the Paleogene.

Seabirds generally live longer, breed later and have fewer young than other birds, but they invest a great deal of time in their young. Most species nest in colonies, varying in size from a few dozen birds to millions. Many species are famous for undertaking long annual migrations, crossing the equator or circumnavigating the Earth in some cases. They feed both at the ocean's surface and below it, and even on each other. Seabirds can be highly pelagic, coastal, or in some cases spend a part of the year away from the sea entirely.

Seabirds and humans have a long history together: They have provided food to hunters, guided fishermen to fishing stocks, and led sailors to land. Many species are currently threatened by human activities such as oil spills, nets, climate change and severe weather. Conservation efforts include the establishment of wildlife refuges and adjustments to fishing techniques.

There exists no single definition of which groups, families and species are seabirds, and most definitions are in some way arbitrary. Elizabeth Shreiber and Joanna Burger, two seabird scientists, said, "The one common characteristic that all seabirds share is that they feed in saltwater; but, as seems to be true with any statement in biology, some do not." However, by convention, all of the Sphenisciformes (penguins), all of the Phaethontiformes (tropicbirds), all of the Procellariiformes (albatrosses and petrels), all of the Suliformes (gannets, boobies, frigatebirds, and cormorants) except the darters, one family of the Pelecaniformes (pelicans), and some of the Charadriiformes (gulls, skuas, terns, auks, and skimmers) are classified as seabirds. The phalaropes are usually included as well, since although they are waders ("shorebirds" in North America), two of the three species (red and red-necked) are oceanic for nine months of the year, crossing the equator to feed pelagically.

Loons and grebes, which nest on lakes but winter at sea, are usually categorized as water birds, not seabirds. Although there are a number of sea ducks in the family Anatidae that are truly marine in the winter, by convention they are usually excluded from the seabird grouping. Many herons and waders (or shorebirds), such as crab-plovers, are also highly marine, living on the sea's edge (coast), but are also not treated as seabirds. Fish-eating birds of prey, such as sea eagles and ospreys, are also typically excluded, however tied to marine environments they may be. Some birds, such as darters, are primarily found in freshwater habitats, but may occasionally venture into marine or coastal areas as well; such birds are generally not considered to be seabirds.

German ornithologist Gerald Mayr defined the "core waterbird" clade Aequornithes in 2010. This lineage gives rise to the Procellariiformes, Sphenisciformes, Suliformes, Pelecaniformes, Ciconiiformes (not seabirds), and Gaviiformes (not seabirds). The tropicbirds (Phaethontiformes) are part of the Eurypygimorphae lineage, which is sister to the Aequornithes; this clade also includes the non-seabird Eurypygiformes (kagu and sunbittern). The Charadriiformes are more distantly related to the other seabirds, being more closely related to the non-seabird Gruiformes (rails and cranes) and Opisthocomiformes (hoatzin) in the clade Gruae.

Seabirds, by virtue of living in a geologically depositional environment (that is, in the sea where sediments are readily laid down), are well represented in the fossil record. They are first known to occur in the Cretaceous period, the earliest being the Hesperornithes. These were flightless seabirds that could dive in a fashion similar to grebes and loons (using its feet to move underwater), but had beaks filled with sharp teeth. Other Cretaceous seabirds included the gull-like Ichthyornithes. Flying Cretaceous seabirds do not exceed wingspans of two meters; piscivorous pterosaurs occupied seagoing niches above this size.

While Hesperornis is not thought to have left descendants, the earliest modern seabirds also occurred in the Cretaceous, with a species called Tytthostonyx glauconiticus, which has features suggestive of Procellariiformes and Fregatidae. As a clade, the Aequornithes either became seabirds in a single transition in the Cretaceous or some lineages such as pelicans and frigatebirds adapted to sea living independently from freshwater-dwelling ancestors. In the Paleogene both pterosaurs and marine reptiles became extinct, allowing seabirds to expand ecologically. These post-extinction seas were dominated by early Procellariidae, giant penguins and two extinct families, the Pelagornithidae and the Plotopteridae (a group of large seabirds that looked like the penguins). Modern genera began their wide radiation in the Miocene, although the genus Puffinus (which includes today's Manx shearwater and sooty shearwater) might date back to the Oligocene. Within the Charadriiformes, the gulls and allies (Lari) became seabirds in the late Eocene, and then waders in the middle Miocene (Langhian). The highest diversity of seabirds apparently existed during the Late Miocene and the Pliocene. At the end of the latter, the oceanic food web had undergone a period of upheaval due to extinction of considerable numbers of marine species; subsequently, the spread of marine mammals seems to have prevented seabirds from reaching their erstwhile diversity.^{[needs update]}