Sigma Phi Society (ΣΦ) was founded on March 4, 1827 on the campus of Union College as a part of the Union Triad in Schenectady, New York. It is the second Greek fraternal organization founded in the United States. The Sigma Phi Society was the first Greek organization to establish a chapter at another college, which occurred with the founding of the Beta of New York at Hamilton College in 1831, thus making it the first National Greek Organization. The practices and rituals of Sigma Phi are relatively unknown due to its establishment, and continued consideration, as a secret society. Also of note, the Alpha Chapter of Sigma Phi located at Union College has been in continuous operation since 1827 making it the oldest continuously running fraternity chapter in the United States. 
Sigma Phi is a member of the North-American Interfraternity Conference.
Some of these own buildings on the National Register of Historic Places. Chapters italicized are dormant.
- Thomas Fielder Bowie - One of the founding members of Sigma Phi and member of the thirty fourth and thirty fifth Congress of the United States.
- Sen. Elihu Root - U.S. Secretary of War and U.S. Secretary of State in the administration of Theodore Roosevelt. Root was a Nobel Peace Prize Winner, and U.S. Senator.
- Earl Warren - Chief Justice of the U.S. Supreme Court, Governor of California.
- James Schoolcraft Sherman - Vice President under William Howard Taft.
- Arthur C. Nielsen Jr., son of Arthur C. Nielsen creator of Nielsen ratings (both were Sigma Phi members).
- Henry Rathbone, major in the U.S. army during the Civil War and present in Lincoln's booth at Ford's Theatre; was stabbed tackling John Wilkes Booth.
- Elias Peissner, professor of German and Political Economy at Union College, died "at the head of his men" at the Battle of Chancellorsville during the Civil War.
- Hon. Ken Dryden - Hockey Hall of Fame Goaltender and Canadian Member of Parliament.
- Rep. Hastings Keith - United States Congressman from Massachusetts.
- Rep. John Cochrane - United States Congressman, Attorney General of New York, and Brigadier General in the Civil War.
- Philip Will, Jr, co-founder of architecture firm Perkins+Will and President of the American Institute of Architects.
- Ward Wettlaufer, amateur golfer.
- George Grenville Benedict, American Civil War hero and Medal of Honor recipient.
- Daniel Butterfield, American Civil War hero, composer of Taps bugle call.
- Jay Walker, founder of Priceline.com
- Larry Tanenbaum, Chairman of MLSE, owner of the Toronto Raptors and Toronto Maple Leafs
- John Bigelow, US diplomat to France under Lincoln, NY Secretary of State, published The Autobiography of Benjamin Franklin, helped found the New York Public Library
- Tully Banta-Cain, American Football linebacker for the New England Patriots
- Robert S. Harrison, Rhodes Scholar, retired partner at Goldman Sachs, Chairman of the Board of Trustees of Cornell University, CEO of the Clinton Global Initiative
- Seth Flowerman, Entrepreneur, CEO of Career Explorations
- Todd Parent, Founder and CEO of Extreme Pizza
- Patrick Doyle, CEO of Domino's Pizza
- Bob Palffy, Principal, Cresa
- Remo Jacuzzi, CEO of hot tub company Jason International Company
- Charlie Munger, Vice-Chairman of Berkshire Hathaway
- Nat Faxon, actor, comedian and Academy Award-winning screenwriter
- ^ a b c Sigma Phi Society public website, accessed 15 May 2014.
- ^ "What's The Oldest Continually-Running Fraternity Chapter In America?". Total Frat Move. Retrieved 2017-08-16.
- ^ Anson, Jack L.; Marchenasi, Robert F., eds. (1991) . Baird's Manual of American Fraternities (20th ed.). Indianapolis, IN: Baird's Manual Foundation, Inc. p. III-132–133. ISBN 978-0963715906.
- ^ Jessup, Philip C., Elihu Root. Vol. I, 1845-1909; Vol. II, 1905-1937. New York, Dodd, Mead, 1938.