Gaylord Anton Nelson (June 4, 1916 – July 3, 2005) was an American politician and environmentalist from Wisconsin who served as a United States senator and governor. He was a member of the Democratic Party and the founder of Earth Day, which launched a new wave of environmental activism.

Nelson was born in 1916 in Clear Lake, Wisconsin, the son of Mary (Bradt), a nurse, and Anton Nelson, a country doctor. He had Norwegian and Irish ancestry. He grew up and was educated in the local public schools. In 1939, he received a bachelor's in political science at what is now San Jose State University in San Jose, California. In 1942, he received an LL.B. degree from the University of Wisconsin Law School in Madison and was admitted to the bar. He practiced as a lawyer before serving in the United States Army, during which time he saw action in the Okinawa campaign during World War II. He rose to the rank of first lieutenant.

In 1948, Nelson was elected to the Wisconsin Senate. He remained there until 1958, when he was elected governor of Wisconsin. He served for four years as governor, in two two-year terms, before being elected to the United States Senate in 1962. He served three consecutive terms as a senator from 1963 to 1981. In 1963, he convinced President John F. Kennedy to take a national speaking tour to discuss conservation issues. Senator Nelson founded Earth Day, which began as a teach-in about environmental issues on April 22, 1970. During Congressional debate on air pollution and emissions from automobiles in 1970, Nelson also sponsored an amendment to the Clean Air Act which would have phased out gasoline-powered automobiles in favor of electric or steam-powered vehicles. The bill was defeated due to lobbying from automotive manufacturers.

During his 1968 re-election campaign, Nelson was praised by Vince Lombardi, the General Manager and former coach of the Green Bay Packers, as the "nation's #1 conservationist" at a banquet in Oshkosh. Nelson's campaign turned Lombardi's banquet speech into a radio and television campaign commercial, infuriating Lombardi, the Wisconsin Republican Party, and Vince's wife, Marie, who was a staunch Republican.

Although known primarily for his environmental work, Nelson also was a leading consumer advocate, strong supporter of civil rights and civil liberties, and one of the early outspoken opponents of the Vietnam War. In 1969, Nelson was one of four senators to introduce a bill to establish the Wisconsin Islands Wilderness.

In 1970, Nelson called for Congressional hearings on the safety of combined oral contraceptive pills, which were famously called "The Nelson Pill Hearings." As a result of the hearings, side-effect disclosure in patient inserts was required for the pill – the first such disclosure for a pharmaceutical drug.

Nelson further attempted to overhaul how pharmaceuticals were marketed, proposing a bill that would require drug manufacturers to prove both the safety and efficacy of their marketed drugs in 1971. His office received a memorandum in 1975 stating that both Sominex and its competitor Compoz had been shown ineffective when compared to placebos, in spite of the US$34 million spent on sleeping aids in 1974.

Nelson was also a noted advocate of small business. While chairman of the Senate Small Business Committee, he led successful efforts to authorize the first modern White House Conference on Small Business, create the system of Small Business Development Centers at U.S. universities, and improve the way that federal agencies regulate small businesses and other small entities, the Regulatory Flexibility Act.