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.

Eaton was born on December 27, 1883, on a farm near the village of Pugwash in Cumberland County, Nova Scotia, Canada. Besides farming, his father, Joseph Howe Eaton, ran a small general store and the district post office. Cyrus's uncle was Charles Aubrey Eaton who led a Cleveland congregation that included Mr. and Mrs. John D. Rockefeller Sr. who Cyrus Eaton met in 1901 when he was 17 and later became his protégé after Rockefeller hired the young Eaton to be a messenger in Rockefeller's private telegraph room.

Eaton left Nova Scotia in 1899 to attend Woodstock College, a Baptist-affiliated prep school in Woodstock, Ontario. Later he enrolled at McMaster University, a Baptist university, then located in Toronto, where he studied philosophy and finance, intending to enter the Baptist ministry. He earned his bachelor of arts degree in 1905 with a major in philosophy.

After graduating from McMaster he moved to Cleveland and went to work for the East Ohio Gas Company. This was one of many businesses associated with John D. Rockefeller. After working with East Ohio Gas and Rockefeller for two years, he established his own business in 1907, developing gas utilities which at the time were relatively underdeveloped and unconsolidated in Canada. He managed to secure natural-gas franchises in Manitoba, Canada, representing a group of New York investors. The syndicate was unable to complete its financing and went defunct. However, the Manitoba government was sufficiently impressed to allow Eaton to retain the franchises. Eaton formed a new holding company, the Canada Gas & Electric Corp, later consolidated into the Continental Gas & Electric Corp. in 1913.

After spending several years travelling, Eaton settled in Cleveland in 1913 and became active in many businesses. Eaton joined the Otis & Co. banking firm in 1916. In 1926 he established the investment vehicle Continental Shares, Inc., a closed end trust. In 1927 he formed Republic Steel, the 3rd-largest U.S. steel company. His business had a complex structure which some felt to be too highly leveraged. His 1929 wealth was an estimated $100 million, most of which was lost in the Great Depression.

Eaton rebuilt his fortune in the 1940s and 1950s, becoming a director (1943), then board chairman (1954), of the Chesapeake and Ohio Railway, and also board chairman of the West Kentucky Coal Co. (1953).

Eaton married twice. First, in 1907, Margaret House (1887–1956); then Anne Kinder Jones (1922–1992) in 1957. He had seven children: Margaret Grace, Mary Adelle, Elizabeth Ann, Anna Bishop, Cyrus S. Jr., Augusta Farlee, and MacPherson.