William Orville Douglas was an American jurist who served as an associate justice of the Supreme Court of the United States from 1939 to 1975. Douglas was known for his strong progressive and civil libertarian views and is often cited as the U.S. Supreme Court's most liberal justice ever. Nominated by President Franklin D. Roosevelt in 1939, Douglas was confirmed at the age of 40, becoming one of the youngest justices appointed to the court. In 1975, Time called Douglas "the most doctrinaire and committed civil libertarian ever to sit on the court." He is the longest-serving justice in history, having served for 36 years and 211 days.

An associate justice of the Supreme Court of the United States is a justice of the Supreme Court of the United States, other than the chief justice of the United States. The number of associate justices is eight, as set by the Judiciary Act of 1869.