Marshosaurus is a genus of medium-sized carnivorous theropod dinosaur, belonging to the family Piatnitzkysauridae, from the Late Jurassic Morrison Formation of Utah and possibly Colorado.

The generic name honoured the nineteenth century paleontologist Professor Othniel Charles Marsh, who described many dinosaur fossils during the Bone Wars.

[1] The holotype, UMNH VP 6373, was found in a layer of the Brushy Basin Member of the Morrison Formation dating from the late Kimmeridgian, approximately 155 - 152 mya.

[8] In a 2001 study conducted by Bruce Rothschild and other paleontologists, five foot bones referred to Marshosaurus were examined for signs of stress fracture, but none were found.

The Morrison Basin where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west.

In 1877 this formation became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope.

The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as Camarasaurus, Brachiosaurus, Barosaurus, Diplodocus, and Apatosaurus.

The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers.