Procellariiformes /prɒsɛˈlɛəri.ɪfɔːrmiːz/ is an order of seabirds that comprises four families: the albatrosses, the petrels and shearwaters, and two families of storm petrels. Formerly called Tubinares and still called tubenoses in English, procellariiforms are often referred to collectively as the petrels, a term that has been applied to all members of the order, or more commonly all the families except the albatrosses. They are almost exclusively pelagic (feeding in the open ocean), and have a cosmopolitan distribution across the world's oceans, with the highest diversity being around New Zealand.

Procellariiforms are colonial, mostly nesting on remote, predator-free islands. The larger species nest on the surface, while most smaller species nest in natural cavities and burrows. They exhibit strong philopatry, returning to their natal colony to breed and returning to the same nesting site over many years. Procellariiforms are monogamous and form long-term pair bonds that are formed over several years and may last for the life of the pair. A single egg is laid per nesting attempt, and usually a single nesting attempt is made per year, although the larger albatrosses may only nest once every two years. Both parents participate in incubation and chick rearing. Incubation times are long compared to other birds, as are fledging periods. Once a chick has fledged there is no further parental care.

Procellariiforms have had a long relationship with humans. They have been important food sources for many people, and continue to be hunted as such in some parts of the world. The albatrosses in particular have been the subject of numerous cultural depictions. Procellariiforms include some of the most endangered bird taxa, with many species threatened with extinction due to introduced predators in their breeding colonies, marine pollution and the danger of fisheries by-catch. Scientists, conservationists, fishermen, and governments around the world are working to reduce the threats posed to them, and these efforts have led to the signing of the Agreement on the Conservation of Albatrosses and Petrels, a legally binding international treaty signed in 2001.

The order was named Procellariiformes by German anatomist Max Fürbringer in 1888. The word comes from the Latin word procella, which means a violent wind or a storm, and -iformes for order. Until the beginning of the 20th century, the family Hydrobatidae was named Procellariidae, and the family now called Procellariidae was rendered "Puffinidae." The order itself was called Tubinares. A major early work on this group is Frederick DuCane Godman's Monograph of the Petrels, five fascicles, 1907–1910, with figures by John Gerrard Keulemans.

In the Sibley-Ahlquist taxonomy, the tubenoses were included in a greatly enlarged order "Ciconiiformes". This taxonomic treatment was almost certainly erroneous, but its assumption of a close evolutionary relationship with other "higher waterbirds" – such as loons (Gaviiformes) and penguins (Sphenisciformes) – appears to be correct. The procellariiforms are most closely related to penguins, having diverged from them about 60 million years ago.

The diving petrels in the genus Pelecanoides were formerly placed in their own family Pelecanoididae. When genetic studies found that they were embedded within the family Procellariidae, the two families were merged.

All the storm petrels were once placed in the family Hydrobatidae but genetic data indicated that Hydrobatidae consisted of two deeply divergent clades that were not sister taxa. In 2018 the austral storm petrels were moved to the new family Oceanitidae. The northern storm petrels in the family Hydrobatidae are more closely related to the family Procellariidae than they are to the austral storm petrels in the family Oceanitidae.

Earlier molecular phylogenetic studies found the family Oceantidae containing the austral storm petrels as the most basal with differing branching topologies for other three families. More recent large-scale studies have found a consistent pattern with the albatross family Diomedeidae as the most basal and Hydrobatidae sister to Procellariidae.