Leiorhynchus is an extinct genus of brachiopod belonging to the order Rhynchonellida and family Leiorhynchidae. Specimens have been found in South America, North America, and Russia in beds of middle Devonian to Mississippian age (372 to 325 million years old). The genus may have been adapted to dysaerobic environments, colonizing areas of reduced oxygen concentrations rich in organic matter. The genus has been used as an index fossil in North America.