Tropicbirds are a family, Phaethontidae, of tropical pelagic seabirds. They are the sole living representatives of the order Phaethontiformes. For many years they were considered part of the Pelecaniformes, but genetics indicates they are most closely related to the Eurypygiformes. There are three species in one genus, Phaethon. The scientific names are derived from Ancient Greek phaethon, "sun". They have predominantly white plumage with elongated tail feathers and small feeble legs and feet.

The genus Phaethon was introduced in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae. The name is from Ancient Greek phaethōn meaning "sun". The type species was designated as the red-billed tropicbird (Phaethon aethereus) by George Robert Gray in 1840.

Tropicbirds were traditionally grouped in the order Pelecaniformes, which contained the pelicans, cormorants and shags, darters, gannets and boobies and frigatebirds. In some classifications tropicbirds and frigatebirds were sometimes placed as sister taxa. Still, their own set of peculiarities gave them their suborder. In the Sibley–Ahlquist taxonomy, the Pelecaniformes were united with other groups into a large "Ciconiiformes". More recently, this grouping is massively paraphyletic (missing closer relatives of its distantly related groups) and split again.

Microscopic analysis of eggshell structure by Konstantin Mikhailov in 1995 found that the eggshells of tropicbirds lacked the covering of thick microglobular material of other Pelecaniformes. Jarvis, et al.'s 2014 paper "Whole-genome analyses resolve early branches in the tree of life of modern birds" aligns the tropicbirds most closely with the sunbittern and the kagu of the Eurypygiformes, with these two clades forming the sister group of the "core water birds", the Aequornithes, and the Metaves hypothesis abandoned.

The red-billed tropicbird is basal within the genus. The split between the red-billed tropicbird and the other two tropicbirds is hypothesized to have taken place about six million years ago, with the split between the red-tailed and white-tailed tropicbird taking place about four million years ago.

Phaethusavis and Heliadornis are prehistoric genera of tropicbirds described from fossils.

Tropicbirds range in size from 76 cm to 102 cm in length and 94 cm to 112 cm in wingspan. Their plumage is predominantly white, with elongated central tail feathers. The three species have different combinations of black markings on the face, back, and wings. Their bills are large, powerful and slightly decurved. Their heads are large and their necks are short and thick. They have totipalmate feet (that is, all four toes are connected by a web). The legs of a tropicbird are located far back on their body, making walking impossible, so that they can only move on land by pushing themselves forward with their feet.

The tropicbirds' call is typically a loud, piercing, shrill, but grating whistle, or crackle. These are often given in a rapid series when they are in a display flight at the colony. In old literature they were referred to as boatswain (bo'sun'/bosun) birds due their loud whistling calls.