The Sigma Phi Society (ΣΦ) is an American college fraternity. Established in 1827 at Union College in Schenectady, New York, it was the second Greek letter fraternal organization founded in the United States. Sigma Phi was the first collegiate social fraternity to establish a chapter at another college, making it the first national fraternity. It was also a founding member of the National Interfraternity Conference, now the North American Interfraternity Conference. It is part of the Union Triad.

The Sigma Phi Society was formed as a secret society on March 4, 1827, on the campus of Union College in Schenectady, New York. Its founders were Charles Thorn Cromwell, John Thomas Bowie, Thomas Fielder Bowie, and Thomas Sydenham Witherspoon.

The Alpha chapter of Sigma Phi at Union College has been in continuous operation since its founding, making it the oldest continuously running fraternity chapter in the United States. In 1831, Beta of New York was established at Hamilton College, making the society the first Greek fraternal organization in the United States to establish a chapter at another college, thus becoming the first national Greek letter organization in the United States.

In 1834, Sigma Phi became the first fraternity to publish a catalogue of its membership. The fraternity was incorporated in the State of New York in 1885 and was reincorporated in 1920.

In 1901, Sigma Phi became a founding member of the National Interfraternity Conference, now the North American Interfraternity Conference.

Historically, the fraternity had been conservative in adding chapters. In 1963, the fraternity had eleven active chapters, two inactive chapters, and 3,910 living members.

Its headquarters is in Tucson, Arizona.

The practices and rituals of the Sigma Phi Society are relatively unknown due to its establishment, and continued consideration, as a secret society.