Theta Chi (ΘΧ) is an international men's college fraternity. It was founded on April 10, 1856, at Norwich University then-located in Norwich, Vermont. It has initiated more than 215,000 members and has over 8,900 collegiate members across North America.

Theta Chi was founded on April 10, 1856, at Norwich University in Norwich, Vermont, by two military cadets, Frederick Norton Freeman and Arthur Chase. A third man, Egbert Phelps, is considered to be the "assistant founder" for lending his help and advice to Freeman and Chase after transferring to Union College in 1854 (he was a member of Chi Psi fraternity). The first initiates after the founders were Edward Bancroft Williston and Lorenzo Potter, both initiated on April 11, 1856.

When Freeman and Chase founded Theta Chi, they spelled out its purpose in its constitution. Article I stated that the objects of Theta Chi were to "bind by closer bonds the members to each other and the mutual assistance of each of its members;" "the advancement and carrying out of any measures at the institution in which it shall be established which shall be of importance to its members," and "the mutual benefit and improvement of all its members."

Theta Chi's early history is closely connected to the history of Norwich University. In 1866, a fire devastated the university, destroying the Old South Barracks, where the fraternity had been founded. This disaster prompted the university to move from Norwich to its present location in Northfield, Vermont. In 1873, the university nearly closed due to financial reasons; however, fraternity member Charles Dole, then serving as a professor at the university, met all of the university's financial obligations from his own pocket, saving the institution. In the fall of 1881, Norwich University was reduced to only twelve students and Theta Chi's membership was reduced to one collegiate member, James M. Holland. In November of that year, Phil S. Randall and Henry B. Hersey approached Holland and insisted that they be allowed to join Theta Chi; Holland agreed, which saved the fraternity from extinction.

With the help of member Charles Dole, who was serving in the Vermont State Legislature, Theta Chi was incorporated under the laws of Vermont on November 22, 1888. Alpha chapter acquired its first chapter house in 1890. There were early efforts to expand Theta Chi to Dartmouth College and Union College, but the anti-expansion sentiment among members of the Alpha chapter and unstable conditions at the university forced it to remain a single entity for 46 years.

On December 13, 1902, the Beta chapter was installed at the Massachusetts Institute of Technology after member Park Valentine Perkins transferred there from Norwich and petitioned Alpha for a charter. A Grand Chapter was organized in 1908 to direct the fraternity and promote its growth.

On April 14, 1942, Theta Chi's chapters merged with Beta Kappa with the exception of the chapter at Georgia Tech in Atlanta, which chose to become a chapter of Lambda Chi Alpha. The merger brought sixteen collegiate chapters and over 6,000 collegiate and alumnus members into the fraternity's ranks. Unlike other men's college fraternity mergers, Beta Kappa was completely absorbed into Theta Chi with no changes to the name or ritual.

The Foundation Chapter of Theta Chi Fraternity, Inc. was established in 1953 as a charity to provide educational scholarships and assistance. In 1965, the Zeta Gamma chapter was installed at the University of Alberta in Edmonton, Alberta, Canada, making Theta Chi an international fraternity.