Delta Sigma Phi (ΔΣΦ), commonly known as Delta Sig, is a fraternity established in 1899 at The City College of New York (CCNY). It was the first fraternity to be founded based on religious and ethnic acceptance. It is also one of three fraternities founded at CCNY (now a part of the City University of New York (CUNY)).

Since its inception, Delta Sigma Phi has chartered chapters at 233 different colleges and universities, with 106 actively operating undergraduate chapters and colonies across the United States as of 2023. The fraternity has more than 6,000 undergraduate members and more than 120,000 living alumni members. More than 150,000 men have been initiated into Delta Sigma Phi since its founding.

Delta Sigma Phi is a charter member of the North American Interfraternity Conference. Its national headquarters are located in Indianapolis, Indiana, at the Fairbanks Mansion, the former home of Vice President Charles Warren Fairbanks.

At the end of the nineteenth century, most fraternities were exclusively Christian or Jewish and barred membership to individuals based on religion. When a group of friends at the City College of New York tried to join a fraternity, they were denied membership because their group was composed of Christians and Jews. In response, they organized the first Delta Sigma Phi chapter on December 10, 1899. The chapter was called Insula due to its location in Manhattan.

In late 1902, with five members from Insula signing incorporation papers, Delta Sigma Phi was incorporated with the purpose of spreading "the principles of friendship and brotherhood among college men, without respect to race or creed." By 1903 the fraternity had established chapters at Columbia University and New York University.

Delta Sigma Phi recognizes Charles A. Tonsor Jr. (Christian) and Meyer Boskey (Jewish) as its two primary founding fathers. Although Boskey was one of the original members at the City College of New York and Tonsor was one of the charter members of the chapter at New York University, it is believed the fraternity first was developed by a group of nearly a dozen men. During the short period when men of Jewish faith were barred from membership, many of the fraternity's founding documents were ruined. Given the circumstances, the national organization adopted both Boskey and Tonsor as the "founders" given their lifelong commitment to the fraternity and their service as visionaries for the development of the fraternity's ritual and national expansion.

In the two years after the 1914 Convention, Delta Sigma Phi almost doubled in size with the addition of ten chapters. In 1915, the first West Coast chapter, Hilgard chapter at UC Berkeley was installed. Hilgard Chapter was named after a dean at the university and is the fraternity's only chapter without a Greek letter designation, taking the place of the Xi chapter.

Also in 1914, the fraternity decided to admit only white men of the Christian faith, thus rejecting the founders' vision. Many Jewish members and other minorities left Delta Sigma Phi or joined others, including Meyer Boskey, who withdrew active participation in the fraternity for an extended period.