{{Infobox Fraternity |
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}} Founded in 1918, the Upsilon Sigma Phi (ΥΣΦ) is the oldest Greek-letter fraternity in Asia. It is the oldest student organization in the University of the Philippines that has been in continuous existence since its founding and predates the Philippine Collegian which was established in 1922, and the UP Student Council which was revived in 1924. It is also an exclusive fraternity where membership is by invitation only.
It was formally organized on November 19, 1920 in a meeting held at the Metropolitan Restaurant in Intramuros. Four months later, on March 24, 1921, the Greek letters ΥΣΦ standing for the initials of the name "University Students Fraternity" was formally adopted. In the same year, the fraternity also completed its organization with rituals prepared by Graciano Q. Rico, motto (We gather light to scatter), colors (cardinal red, old blue). The head is known as the Illustrious Fellow and the first honorary fellow, University Regent Conrado Benitez, was inducted into the Fraternity. He wrote the Upsilon Hymn which later would be sung before and after every formal meeting.
The Upsilon Sigma Phi has two chapters. A combined UP Diliman/UP Manila chapter and a second one in UP Los Banos.
During World War II, some members took the field. Among the Upsilonians who gave up their lives were Wenceslao Q. Vinzons (former UP Student Council President, youngest delegate to the 1934 Constitutional Convention, and Governor of Camarines), Agapito del Rosario (Mayor of Angeles, Pampanga), and José Abad Santos (Secretary of Justice and Chief Justice of the Supreme Court), all of whom were executed for refusing to swear allegiance to the Japanese Empire.
The administration of President Ferdinand E. Marcos saw Upsilon members lead opposing sides in the leadership of the Philippines. With the administration was Marcos, Senate President Arturo M. Tolentino, Secretary of Education and former UP President Onofre Corpuz, Chief Justice Querube Makalintal (who after his retirement would be elected Speaker of the Batasang Pambansa), and Batasan Speaker Nicanor Yniguez among many others. Leading the opposition were Senators Benigno S. Aquino, Jr., Gerardo Roxas, Salvador H. Laurel and Mamintal A.J. Tamano.