Arnold L. Demain (April 26, 1927 – April 3, 2020) was an American microbiologist. During a 60-year career, he gained a reputation in the field of industrial microbiology. He was the Professor of Industrial Microbiology in the Biology Department at MIT and Founder and Head of Department of Fermentation Microbiology at Merck & Co. The August 2010 edition of The Journal of Antibiotics celebrated his scientific career. Demain was described as “one of the world’s leading industrial microbiologists” and as “a scientist constantly in the forefront of industrial microbiology and biotechnology.” He was “a pioneer in research on the elucidation and regulation of the biosynthetic pathways leading to the penicillins and cephalosporins” and "instrumental in the development of the beta-lactam industry".