Othniel Charles Marsh (October 29, 1831 - March 18, 1899) was one of the pre-eminent paleontologists of the 19th century, who discovered and named many fossils found in the American West.

Marsh was born in Lockport, New York. He graduated at Yale College in 1860, and studied geology and mineralogy in the Sheffield scientific school, New Haven, and afterwards palaeontology and anatomy in Berlin, Heidelberg and Breslau. He returned to the United States in 1866 and was appointed professor of vertebrate paleontology at Yale University. He persuaded his uncle, George Peabody, to establish the Peabody Museum of Natural History at Yale.

In May 1871 Marsh found the first American pterosaur fossils. He also discovered the remains of early horses. Marsh described the remains of Cretaceous toothed birds (such as Ichthyornis and Hesperornis) and flying reptiles, and Cretaceous and Jurassic dinosaurs, including Apatosaurus and Allosaurus.

Marsh is famous for his "palaeontological battle", the so-called Bone Wars, with Edward Drinker Cope during the late 19th century. The two men were fierce rivals in the discovery of palaeontological specimens, discovering and describing over 120 new species of dinosaur between them.

Marsh is interred at the Grove Street Cemetery, New Haven, Connecticut.