Alfred Rehder (4 September 1863 in Waldenburg, Saxony – 25 July 1949 in Jamaica Plain, Massachusetts) was a German-American botanical taxonomist and dendrologist who worked at the Arnold Arboretum of Harvard University. He is generally regarded as the foremost dendrologist of his generation.

Georg Alfred Rehder was born in the castle of Waldenburg to Thekla née Schmidt (1839–1897) and Paul Julius Rehder (1833–1917), the superintendent of parks and gardens of the principality of Schönburg-Waldenburg. Through his father, Rehder was introduced to the gardening profession. On his mother's side of the family, Rehder was likely descended from Henry, Duke of Anhalt-Köthen (1778–1847).

Rehder broke off his attendance at the gymnasium in Zwickau in 1881 and did not pursue university studies, instead working for three years as an apprentice under the tutelage of his father. His professional career began in 1884 at the Berlin Botanical Garden. Here he was able to attend lectures by Paul Friedrich August Ascherson and August Wilhelm Eichler, among others. In 1886, he went to work for a florist in Frankfurt am Main, and half a year later he moved to the Muskau Park. Here he first met the daughter of the local parks superintendent, Anneliese Hedwig Schrefeld (1875–1967), whom he married in 1906.

In 1888, Rehder accepted an appointment as head gardener at the Darmstadt Botanical Garden. He moved next to the Göttingen Botanical Garden, where he was head gardener from 1889 to 1895. During this time, he was involved in the creation of the Brocken Garden for Alpine plants, initiated by Albert Peter in 1890 on the highest mountain of the Harz range. In addition, he became a contributing editor of several professional periodicals. In 1895, he was appointed associate editor of Möller's Deutsche Gärtner-Zeitung (published in Erfurt), Germany's premier horticultural journal, for which he wrote numerous articles.

In 1898, he was assigned by the Deutsche Gärtner-Zeitung to travel to the United States and study woody plants and orchards. The German government also asked him to research American grape species, known to be resistant to the grape phylloxera that were threatening to infest German vineyards and destroy the wine industry. He undertook research at the Arnold Arboretum, and here he came to the attention of the director, Charles Sprague Sargent, who quickly recognized Rehder's abilities and persuaded him to stay at the arboretum and work on the comprehensive study of woody plants eventually published as The Bradley Bibliography (5 vols., 1911–18). At about this time, he was also introduced to Liberty Hyde Bailey of Cornell University, who asked him to prepare the text on woody genera for the Cyclopedia of American Horticulture (4 vols., 1900–02).

Rehder was naturalized as an American citizen in 1904, but he maintained his ties with Germany. During World War I, he was subject to surveillance by the Bureau of Investigation.

Collaborating closely with Charles Sprague Sargent, Rehder launched the Journal of the Arnold Arboretum, which appeared as a quarterly from 1919 to 1990. The journal had a particular focus on dendrology but also covered other botanical fields. He was instrumental in systematizing the thousands of plants collected by Ernest Henry Wilson in China.

Rehder created the first system of isothermic zones for the United States that related average winter minimum temperatures to the hardiness of specific plants. The system, along with another developed by Wladimir Köppen, is the basis for the USDA Hardiness zone maps in use today.