Denis Hayes (1944- ) has been an important environmental activist and proponent of Solar power. He rose to prominence in 1970 as the activities coordinator for the first Earth Day.

Denis Hayes was born in Camas, Washington, in 1944. His experiences growing up in the Pacific Nortwest instilled a lifelong love of nature. As a high-school graduate, he enrolled in Clark College. After two years, deciding he wanted to see a bit more of the world, he worked odd jobs. After a while, he enrolled as a history major at Stanford University. This was during the Vietnam War years, and he became an activist opposing that war. Graduating from Stanford, Hayes continued studies in Harvard Law School.

Around this time, U.S. Senator Gaylord Nelson had come up with the idea of a national holiday celebrating conservation and environmental concerns. Hayes left school again (in 1970), with Senator Nelson having appointed him as the activities coordinator who would be a chief organizer of the first Earth Day. The first Earth Day (April 22, 1970) had participants and celebrants in two thousand colleges and universities, about ten thousand primary and secondary schools, and hundreds of communities. It is believed that some 20 million demonstrators participated.

After this, Denis Hayes became head of the Solar Energy Research Institute, but left this position when the Reagan administration cut funding for the program. Hayes went back to school and completed a law degree at Stanford, and went on to become an adjunct professor of engineering in that university.

Hayes served as international chairman for Earth Day's twentieth anniversary in 1990. This time, the event involved 141 countries and (it is estimated) over 200 million people. Since then, Hayes has been president of the Bullitt Foundation and served as chairman of the board of Green Seal. Denis Hayes has published over 100 articles and books.