any cetacean possessing unique epidermal modifications of the mouth called baleen, which is used to filter food from water.

Baleen whales seek out concentrations of small planktonic (plankton) animals. The whales then open their mouth and take in enormous quantities of water. When the mouth is closed, they squeeze the water out through the sides, catching the tiny prey on the baleen's bristles (see also cetacean: Feeding adaptations (cetacean)).

Baleen (whalebone) is a keratinized (keratin) structure like hair, fingernails, and hooves. The baleen apparatus hangs down in two transverse rows, one from each side of the roof of the mouth ( palate). Each row contains up to 400 elongated, triangular plates. The longest sides of the plates are smooth and situated along the outer edge of the mouth, whereas the inner sides are frayed into bristles. In the Greenland right whale (Balaena mysticetus), single plates of baleen can reach 5.2 metres (17 feet) long. Before the invention of spring steel and celluloid in the 19th century, “whalebone,” as baleen was called, was very valuable. Because it is flexible and retains shapes imposed on it with heat, baleen was used for springs and in products such as corsets, knife handles, umbrella ribs, brushes, and fans.

Baleen whales evolved from ancestors that had teeth. Some of the early mysticetes had baleen on the palate in addition to a few functional teeth. The name of the mysticete suborder is derived from the Greek mystax, referring to the baleen as a “mustache,” and ketos (Latin cetus), meaning “whale.”

For a taxonomy of suborder mysticeti, see cetacean: Classification and paleontology (cetacean). For information on specific baleen whales, see blue whale; fin whale; gray whale; humpback whale; right whale; rorqual; sei whale.