Ehrenfried Erwin Pfeiffer (19 February 1899 – 30 November 1961) was a German scientist, soil scientist, leading advocate of biodynamic agriculture, anthroposophist and student of Rudolf Steiner.

Ehrenfried Pfeiffer began work with Rudolf Steiner in 1920 to develop and install special diffuse stage lighting for eurythmy performances on the stage of the first Goetheanum. After Steiner's death in 1925, Pfeiffer worked in the private research laboratory at the Goetheanum in Dornach, (Switzerland). He became manager and director of the 800-acre (3.2 km²) experimental biodynamic Loverendale farm in Domburg in the Netherlands. This farm was set up to carry out some of the agricultural studies of the Goetheanum laboratory. The work of testing and developing Rudolf Steiner's Agriculture Course of 1924 was an international enterprise coordinated by Pfeiffer at the Natural Science Section of the Goetheanum. Pfeiffer’s most influential book 'Bio-Dynamic Farming and Gardening' was published in 1938 simultaneously in at least five languages, English, German, Dutch, French, and Italian. The following year, and just months before the outbreak of World War II, Pfeiffer ran Britain's first biodynamics conference, the Betteshanger Summer School and Conference, at the estate of Lord Northbourne in Kent. Pfeiffer's Betteshanger Conference is regarded as the 'missing link' between biodynamic agriculture and organic farming because the following year (1940) its host, Lord Northbourne, published his manifesto of organic farming 'Look to the Land' in which he coined the term 'organic farming'.

Pfeiffer first visited the U.S. in 1933 to lecture to a group of anthroposophists at the Threefold Farm in Spring Valley, New York on biodynamic farming. His consulting was essential to the development of biodynamic agriculture in the U.S.

Pfeiffer developed an analytical method using copper chloride crystallization and used this technique as a blood test for detecting cancer. As a result, Pfeiffer was invited to the U.S. in 1937 to work at the Hahnemann Medical College in Philadelphia While in the U.S., he continued to consult with those interested in biodynamic farming and helped to form the Biodynamic Farming & Gardening Association in 1938. in 1940 he immigrated to the U.S. from Switzerland with his wife Adelheid, escaping the advance of German troops into France. They brought with them their son Christoph and daughter Wiltraud.

With the advent of World War II in Europe, Pfeiffer took his family to Kimberton, Pennsylvania (near Philadelphia), where Alaric Myrin offered Pfeiffer the opportunity to create a model biodynamic farm and training program. Starting in the late 1930s he taught biodynamic farming and gardening at the Kimberton Farm School. One of his students, Paul Keene, who worked and studied with Pfeiffer there for two years and shortly thereafter co-founded Walnut Acres, recalls: "he helped bring all of life together for us in a definite coherent pattern".

While at Kimberton, Pfeiffer led the initiative to found the Biodynamic Farming and Gardening Association, and to start its journal. While at Kimberton, Pfeiffer also met J. I. Rodale, founder of Organic Gardening and Farming magazine, and of the organic movement in the U.S. This relationship gave biodynamics a little-known place in the history of the American organic movement. Interpersonal difficulties - a motif of Pfeiffer's life - brought to a close the Kimberton Farms chapter.

Aiming to continue his work training biodynamic farmers, Pfeiffer bought a farm in Chester, New York, where a small colony arose focused on farming, education, and the administration of the Biodynamic Association.

His copper chloride sensitive crystallization theory brought him an honorary degree of Doctor of Medicine from Hahnemann Medical College and Hospital in Philadelphia in 1939. He studied chemistry and became a professor of nutrition in 1956. Pfeiffer wrote on the dangers of pesticides and DDT and Rachel Carson consulted with him when she was writing Silent Spring.