Gigantosaurus (meaning "giant lizard") is a poorly known sauropod dinosaur genus from England. The type species, Gigantosaurus megalonyx, was named and described by Harry Govier Seeley in 1869. Its type, BMNH 32498-99, consists of several separately discovered sauropod bones found near Stretham, among which two casts of the "large claw" the specific name, derived from Greek megalos and onyx, refers to. Other remains included a cervical (neck) vertebra, two caudal (tail) vertebrae, the distal end of a tibia, a fibula, and a osteoderm. It was synonymised to Ornithopsis humerocristatus by Richard Lydekker in 1888 and to Pelorosaurus by Friedrich von Huene in 1909. Today it is considered a nomen dubium.
Because of these references Eberhard Fraas, in 1908, incorrectly assumed the name was available for other species and he used it, despite it being preoccupied, for African material, totally unrelated to the British finds. As a result the name Gigantosaurus factored into the convoluted taxonomic history of the African dinosaurs Barosaurus, Tornieria, and Janenschia.