Elmer Drew Merrill (October 15, 1876 – February 25, 1956) was an American botanist and taxonomist. He spent more than twenty years in the Philippines where he became a recognized authority on the flora of the Asia-Pacific region. Through the course of his career he authored nearly 500 publications, described approximately 3,000 new plant species, and amassed over one million herbarium specimens. In addition to his scientific work he was an accomplished administrator, college dean, university professor and editor of scientific journals.

Merrill and his twin brother, Dana, were born and raised in East Auburn, Maine, the youngest of six children born to Daniel C. and Mary (Noyes) Merrill. Merrill showed an early interest in natural history, collecting and identifying plants, birds' eggs, rocks, and minerals. In 1894 he entered the University of Maine with the intention of studying engineering but soon switched to a general science curriculum where he focused on the biology and classification of flowering plants. He was the valedictorian of his graduation class in 1898 and then stayed on for an additional year working as an assistant in the Department of Natural Science. During his time in college Merrill built a sizable herbarium of almost 2,000 specimens which he eventually donated to the New England Botanical Club.

In 1899 Merrill accepted a position with the United States Department of Agriculture in Washington D.C. as an assistant to Frank Lamson-Scribner, an authority on the classification of grasses and a pioneer plant pathologist. At the USDA he learned the principles of plant taxonomy and became proficient in the development and management of a herbarium. His training was supplemented by fieldwork in Idaho, Montana, and Wyoming.^{[citation needed]}

At the end of the Spanish–American War, the United States Taft Commission established the Insular Bureau of Agriculture in Manila. Merrill was appointed to the post of botanist in the new organization and arrived in 1902 in Manila where he was to work for the next twenty-two years. Merrill was dismayed to discover that the herbarium he was expecting to find had been destroyed during the war, along with the botanical library and scientific equipment. Nevertheless, starting with just an empty building, he set out to rebuild the herbarium and library.

Within a few months his role was expanded to include a joint appointment with the Bureau of Forestry. Over the years Merrill's responsibilities continued to grow until he became both the Director of the Bureau of Sciences and a Professor of Botany at the University of the Philippines. He collected and studied plants not only from the Philippines but also from the greater Asia-Pacific region including Indonesia, Malaysia, Indochina, China, and Guam. Eventually the herbarium grew to over 250,000 specimens and the botanical library was recognized as one of the best in Asia. At that time he edited three exsiccata series, the largest with 1200 numbered units under the title Plantae Insularum Philippinensium. Another large series was devoted to plants described by Manuel Blanco Ramos and Antonio Llanos Aller. Together with Charles Budd Robinson he worked on the legacy of Georg Eberhard Rumphius and the interpretation of his Herbarium Amboinense.

Merrill published more than 100 taxonomic papers on Philippine flora and several additional papers on the flora of the region. Many of his papers were published in The Philippine Journal of Science, a journal he helped establish and edited from 1907 to 1918. In 1912, he published a 500-page Flora of Manila covering some 1,000 species. In 1921, he completed a Bibliographic Enumeration of Bornean Plants, a volume of 637 pages. His most ambitious work was the Enumeration of Philippine Flowering Plants published in sections between 1923 and 1926. As documented by his Enumeration the list of known Philippine species had been extended from 2,500 plants of all types in 1900 to 8,120 species of flowering plants, 1,000 species of ferns, and 3,000 species of cryptogams.

In 1924 Merrill returned to the United States to join the University of California, Berkeley. There he was appointed Dean of the College of Agriculture and Director of the Agricultural Experiment Station. At Berkeley he led a reorganization of the faculty, revised the curriculum, emphasized academic training of staff, added buildings and equipment, and stressed fundamental research. In 1925, Merrill established the journal Hilgardia, named for Eugene W. Hilgard who organized the Agriculture Department and was the founding director of the Agricultural Experiment Station. During his spare time, Merrill continued to work on systematic botany of the Asia-Pacific flora and added more than 100,000 specimens from that region to the university herbarium.

In 1926, a proposal was developed to establish the California Botanic Garden in Los Angeles. Merrill took part-time leave from the college to become director of the Garden Foundation. The plan involved the purchase of 4,500 acres (1,800 ha) at Mandeville Canyon in the Santa Monica Mountains. About 800 acres in the center of the tract were to be developed as a botanical garden financed by the sale of surrounding property for residential homes. During his short tenure as director Merrill built administrative offices and greenhouses, started a library, established an herbarium of 180,000 specimens and planted 1,200 species in the gardens. Unfortunately, not long after Merrill left, the plans collapsed when property prices plummeted as a result of the Great Depression. The herbarium was transferred to the University of California, Los Angeles and the gardens were subdivided and sold for housing.