William Emerson Ritter (November 21, 1856 – January 10, 1944) was an American biologist. Ritter initiated and shaped the Marine Biological Association of San Diego (now Scripps Institution of Oceanography of UC San Diego) and the American Society for the Dissemination of Science (now the Society for Science and the Public and Science News). Innovative and entrepreneurial, with a deep desire for human service, he worked tirelessly to educate people in science thinking. He was the first biologist to propose a theory of systems, and seems to be the originator of the term organicism for biological purposes.