The vallakiforms are a clade of zalkine hexapods, related to the draconiforms. Collectively, they bear most resemblance to Earthly plesiosaurs or mythical sea serpents.

Though descended from non-aquatic ancestors, all known vallakiforms spend most or all of their lives in the water. Though several deep-ocean lineages have, via symbionts in their respiratory tracts, developed the ability to extract oxygen directly from water, most species are air-breathing; some, in the manner of the earliest vallakiforms, lay eggs on shore or in intertidal zones, but most species are unable to venture onto land even for brief periods and give live birth in the water.

Most species are marine, living in oceans or in large inland seas, but there are some freshwater species native to large lakes and slow-moving rivers.