Kappa Sigma (ΚΣ), commonly nicknamed Kappa Sig, is an international fraternity with currently 306 active chapters and colonies in North America. Kappa Sigma has initiated more than 240,000 men on college campuses throughout the United States and Canada. Today, the Fraternity has over 175,000 living alumni and more than 14,250 undergraduate members. It is one of the leaders of all American fraternities in terms of pledges and new initiates per year, service hours, and philanthropic donations. Its endowment fund, founded in 1919 is the oldest college fraternity foundation and has donated more than $6.5 million to undergrads since 1948 and now raises over $250,000 per year.

The Kappa Sigma Fraternity states that it is modeled after an extinct order said to have been founded at the University of Bologna in 1400. In some accounts, this order was known as "Kirjath Sepher", founded between 1395 and 1400. According to the traditional story, the corrupt governor of the city, one-time pirate and later papal usurper Baldassare Cossa, took advantage of the students at Bologna, one of Europe's preeminent universities which attracted students from all over the continent, by sending his men to assault and rob them; this motivated one of the university's scholars Manuel Chrysoloras to found a secret society of students beginning with five of his most devoted disciples, for mutual protection against Baldassare Cossa.