Liberty Hyde Bailey (March 15, 1858 – December 25, 1954) was an American horticulturist and reformer of rural life. He was cofounder of the American Society for Horticultural Science. As an energetic reformer during the Progressive Era, he was instrumental in starting agricultural extension services, the 4-H movement, the nature study movement, parcel post and rural electrification. He was considered the father of rural sociology and rural journalism.

Born in South Haven, Michigan, as the third son of farmers Liberty Hyde Bailey Sr. and Sarah Harrison Bailey. In 1876, Bailey met Lucy Millington who encouraged his interest in botany and mentored him. Bailey entered the Michigan Agricultural College (MAC, now Michigan State University) in 1877 and graduated in 1882 (he had taken a year off from study for health reasons). The next year, he became assistant to the renowned botanist Asa Gray, of Harvard University. This was arranged by a professor at MAC, William James Beal. Bailey spent two years with Gray as his herbarium assistant. The same year, he married Annette Smith, the daughter of a Michigan cattle breeder, whom he met at the Michigan Agricultural College. They had two daughters, Sara May, born in 1887, and Ethel Zoe, born in 1889.

In 1884 Bailey returned to MAC to become professor and chair of the Horticulture and Landscape Gardening Department, establishing the first horticulture department in the country.

In 1888, he moved to Cornell University in Ithaca, New York, where he assumed the chair of Practical and Experimental Horticulture. In 1896, he was elected to the American Philosophical Society. He was elected an Associate Fellow of the American Academy of Arts and Sciences in 1900. He founded the College of Agriculture, and in 1904 he was able to secure public funding. He was dean of what was then known as New York State College of Agriculture from 1903 to 1913. In 1908, he was appointed Chairman of The National Commission on Country Life by president Theodore Roosevelt. Its 1909 Report called for rebuilding a great agricultural civilization in America. In 1913, he retired to become a private scholar and devote more time to social and political issues. In 1917 he was elected a member of the United States National Academy of Sciences.

He edited The Cyclopedia of American Agriculture (1907–09), the Cyclopedia of American Horticulture (1900–02) (continued as the Standard Cyclopedia Of Horticulture (1916–1919)) and the Rural Science, Rural Textbook, Gardencraft, and Young Folks Library series of manuals. He was the founding editor of the journals Country Life in America and the Cornell Countryman. He dominated the field of horticultural literature, writing some sixty-five books, which together sold more than a million copies, including scientific works, efforts to explain botany to laypeople, a collection of poetry; edited more than a hundred books by other authors and published at least 1,300 articles and over 100 papers in pure taxonomy.

He coined the words cultivar, cultigen, and indigen. His most significant and lasting contributions were in the botanical study of cultivated plants. Bailey's publisher was George Platt Brett, Sr. of Macmillan Publishers (United States).

Bailey and his family are interred in a grand Egyptian Revival styled mausoleum of his own design at Lake View Cemetery in Ithaca, New York.

Bailey was one of the first to recognize the overall importance of Gregor Mendel's work. He cited Mendel's 1865 and 1869 papers in the bibliography that accompanied his 1892 paper, "Cross Breeding and Hybridizing". Mendel is mentioned again in the 1895 edition of Bailey's "Plant Breeding".