A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, cetaceans (whales, dolphins, and porpoises), and the (extinct) ichthyosaurs. Depending on the species, an animal may have one or two of them.

Wildlife biologists often use the distinctive nicks and wear patterns which develop on the dorsal fins of large cetaceans to identify individuals in the field.

The bony or cartilaginous bones that support the base of the dorsal fin in fishes are called pterygiophores.

The main purpose of the dorsal fin is to stabilize the animal against rolling and to assist in sudden turns. Some species have further adapted their dorsal fins to other uses. The sunfish uses the dorsal fin (and the anal fin) for propulsion. In anglerfish, the anterior of the dorsal fin is modified into a biological equivalent to a fishing pole and a lure known as illicium or esca. Many catfish can lock the leading ray of the dorsal fin in an extended position to discourage predation or to wedge themselves into a crevice. Some animals have developed dorsal fins with protective functions, such as spines or venom. For example, both the spiny dogfish and the Port Jackson shark have spines in their dorsal fins which are capable of secreting poison.