Cyrus Stephen Eaton Sr. (December 27, 1883 – May 9, 1979) was a Canadian-American investment banker, businessman and philanthropist, with a career that spanned 70 years.
For decades Eaton was one of the most powerful financiers in the American Midwest, and he was a colourful and often-controversial figure. He was chiefly known for his longevity in business, for his opposition to the dominance of eastern financiers in the America of his day, for his occasionally ruthless financial manipulations, for his passion for world peace and for his outspoken criticism of United States Cold War policy. He funded and helped organize the first Pugwash Conferences on World Peace, in 1957. He wrote numerous articles and essays on political and economic subjects—"Investment Banking: Competition or Decadence?", "Rationalism Versus Rockefeller", and "A Capitalist Looks at Labour" being some of the best known.