St. George Jackson Mivart (November 30, 1827 - April 1, 1900) was an English biologist.

He was born in London, and educated at Clapham grammar school, Harrow School, and King's College, London, and afterwards at St Mary's, Oscott. (His conversion to Roman Catholicism automatically excluded him from the University of Oxford.) In 1851 he was called to the bar, but he devoted himself to medical and biological studies. In 1862 he was appointed lecturer at St Mary's Hospital medical school, in 1869 he became a fellow of the Zoological Society of London, and from 1874 to 1877 he was professor of biology at the short-lived Roman Catholic University College, London . In 1873 he published Lessons in Elementary Anatomy, and an essay on Man and Apes. In 1881 appeared The Cat: an Introduction to the Study of Back-boned Animals. The careful and detailed work he bestowed on Insectivora and Carnivora largely increased our knowledge of the anatomy of these groups. In 1871 his Genesis af Species brought him into the controversy then raging. Though admitting evolution generally, Mivart denied its applicability to the human intellect.

His views as to the relationship between human nature and intellect and animal nature in general were given in Nature and Thought (1882); and in the Origin of Human Reason (1889) he stated what he considered the fundamental difference between men and animals. In 1884, at the invitation of the Belgian episcopate, he became professor of the philosophy of natural history at the University of Leuven, which had conferred on him the degree of M.D. in 1884. Some articles published in the Nineteenth Century in 1892 and 1893, in which Mivart advocated the claims of science even where they seemed to conflict with religion, were placed on the Index expurgatorius , and other articles in January 1900 led to his excommunication by Cardinal Vaughan, with whom he had a curious correspondence vindicating his claim to hold liberal opinions while remaining in the Roman Catholic Church. Shortly afterwards he died, in London.

Mivart was a critic whom Charles Darwin took most seriously. One of his criticisms, to which Darwin effectively responded in later editions of The Origin of Species, was a perceived failure of natural selection to explain the incipient stages of useful structures.

Mivart was a member of the Metaphysical Society and was also the author of many scientific papers and occasional articles, and of Castle and Manor: a Tale of our Time (1900), which originally appeared in 1894 as Henry Standon, by "D'Arcy Drew."