Asa Gray (November 18, 1810 – January 30, 1888) is considered the most important American botanist of the 19th century.

He was instrumental in unifying the taxonomic knowledge of the plants of North America. Of Gray's many works on botany, the most popular was his Manual of the Botany of the Northern United States, from New England to Wisconsin and South to Ohio and Pennsylvania Inclusive. This book, known simply as Gray's Manual, has gone through a number of editions with botanical illustrations by Isaac Sprague, and remains a standard in the field.

A devout Christian as an adult, Gray had been born in Sauquoit, New York, in 1810, and became an M.D. in 1831. However, he relinquished medicine for botany, and in 1842 was appointed professor of natural history at Harvard University, a post he retained until 1873 while living in the Asa Gray House. Through the donation of an immense book and plant collection numbering in the thousands, he effectively created the botany department at Harvard; the Gray Herbarium is named after him. He was President of the AAAS in 1871. In 1859, he was elected a foreign member of the Royal Swedish Academy of Sciences.

He was a pupil of John Torrey, with whom he worked closely; they published the Flora of North America together. The "Elements of Botany" (1836), an introductory textbook, was the first of Gray's many works.

Gray traveled to the American west on two separate occasions, the first in 1872 and then again with Joseph Dalton Hooker in 1877. Both times his goal was botanical research: he avidly collected plant specimens to bring back with him to Harvard. On his second trip through the American west, he and Hooker reportedly collected over 1000 specimens. They were accompanied for a time by Ferdinand Vandeveer Hayden, then in charge of the U.S. Geological and Geophysical Survey of the Territories. Gray and Hooker's research was reported in their joint 1882 publication, "The Vegetation of the Rocky Mountain Region and a Comparison with that of Other Parts of the World," which appeared in volume six of Hayden's Bulletin of the United States Geological and Geophysical Survey of the Territories.

On both trips he climbed Grays Peak, one of Colorado's many fourteeners. This mountain was named after Gray by the botanist and explorer of the Rocky Mountains Charles Christopher Parry, who was likely a student of Gray's at Harvard.

In 1880 David P. Penhallow was accepted by Gray as a research assistant. Penhallow aided in Grays work regarding the distribution of northern hemisphere plants, and in 1882 Gray recommended Penhallow as a lecturer to Sr John Dawson of McGill University in Montreal.

He died in 1888 and was buried at Mount Auburn Cemetery. The cemetery's Asa Gray Garden, with a central fountain and numerous usual tree varieties, is named in his honor.

Gray and Charles Darwin met at Kew, introduced by Joseph Dalton Hooker. Darwin then wrote to Gray requesting information about the distribution of various species of American flowers, which Gray provided, and which was helpful in providing information for the development of Darwin's theory. This was the beginning of an extensive lifelong correspondence.

Gray, Darwin and Hooker became lifelong friends and colleagues, and Gray and Hooker conducted research on Darwin’s behalf in 1877 on their Rocky Mountain expedition. After Hooker returned to England and reported to Darwin on their adventure, Darwin wrote back to Gray: “I have just... heard prodigies of your strength & activity. That you run up a mountain like a cat!”

When Darwin received Alfred Russel Wallace's paper which described natural selection, Hooker and Charles Lyell arranged for a joint reading of papers by Darwin and Wallace to the Linnean Society. Since Darwin had nothing prepared, the reading included excerpts from his 1844 Essay and from a letter he had sent to Asa Gray in 1857, outlining his theory. The correspondence with Gray was thus a key piece of evidence in establishing Darwin's intellectual priority with respect to the theory of evolution through natural selection. Neither Darwin nor Wallace attended the meeting. The papers were published by the society as On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.

Gray arranged the first US Edition of On the Origin of Species and negotiated royalties on Darwin's behalf. Darwin held Gray in high esteem: he dedicated his book Forms of Flowers to Gray and he wrote in 1881 "there is hardly any one in the world whose approbation I value more highly than I do yours". Gray, considered by Darwin to be his friend and "best advocate", also attempted to convince Darwin in these letters that design was inherent in all forms of life, and to return to his faith. Darwin agreed that his theories were "not at all necessarily atheistical" but was unable to share Gray's belief. "I feel most deeply that the whole subject is too profound for the human intellect. A dog might as well speculate on the mind of Newton," he wrote.

Notwithstanding, Gray was a staunch supporter of Darwin in America, and collected together a number of his own writings to produce an influential book, Darwiniana. These essays argued for a conciliation between Darwinian evolution and the tenets of theism, at a time when many on both sides perceived the two as mutually exclusive. Gray denied that investigation of physical causes stood opposed to the theological view and the study of the harmonies between mind and Nature, and thought it "most presumable that an intellectual conception realized in Nature would be realized through natural agencies"

In 1868 Gray had a year's leave of absence and visited Darwin in England – the first time they had met since they started their correspondence. Darwin had Gray in mind when he wrote that "It seems to me absurd to doubt that a man may be an ardent theist & an evolutionist".

The Asa Gray Award, the highest award of the American Society of Plant Taxonomists, was established in 1984 to honor a living botanist for career achievements. Grayanotoxin and Gray Peak (New York) are named after him. Also, a residential building is named after him on the Stony Brook University campus.