Sigma Chi was founded in 1855 by Benjamin Piatt Runkle, Thomas Cowan Bell, William Lewis Lockwood, Isaac M. Jordan, Daniel William Cooper, Franklin Howard Scobey, and James Parks Caldwell as the result of a disagreement over who would be elected Poet in the Erodelphian Literary Society of Miami University in Ohio.
Several members of Miami University's Delta Kappa Epsilon chapter (of which all but one of Sigma Chi's founders were members) were also members of the Erodelphian Literary Society. In the fall of 1854 the literary society was to elect its Poet and a member of Delta Kappa Epsilon was nominated for the position. He was supported by five of his brothers, but four others (James Caldwell, Isaac Jordan, Benjamin Runkle, and Franklin Scobey) supported another man who was not a member of the fraternity. Although Thomas Bell and Daniel Cooper were not members of Erodelphian they had aligned themselves with the four dissenting members. The chapter had twelve members total and were evenly divided on the issue. Both sides saw this as a matter of principle and over the next few months their friendships became distanced.
In February 1855 Runkle and his companions planned a dinner for their brothers in an attempt to seal the rift. Whitelaw Reid, one of the other brothers who supported the Delta Kappa Epsilon member as poet, was the only one to arrive. Reid brought a Delta Kappa Epsilon alumnus named Minor Millikin from a nearby town. Reid had told Millikin his side of the dispute and they had arrived to punish the group for not supporting their Delta Kappa Epsilon brother. The leaders of the rebellion, Runkle and Scobey, were to be expelled from the fraternity. The other four would be allowed to stay in the fraternity. Runkle resigned, and after the parent chapter at Yale University was contacted, all six men were formally expelled.
The six men decided to form their own fraternity along with William Lewis Lockwood, a student from New York who had not joined a fraternity. On June 28, 1855, the organization was founded under the name Sigma Phi Fraternity. Lockwood used his business training to help organize the fraternity in its early years. The eventual theft of Sigma Phi's constitution, rituals, seals, and other records from Lockwood's room in Oxford in January 1856 prompted them to change the name of the fraternity to Sigma Chi. It is possible this action could have been forced upon the group as there was already a Sigma Phi Society.
Much of Sigma Chi's heraldry was inspired by the legendary story of the Emperor Constantine from the Battle of Milvian Bridge against Maxentius. The White Cross and the motto "In Hoc Signo Vinces" are examples of the Constantine link. Although many of the symbols of Sigma Chi relate to Christianity, Sigma Chi is not a Christian fraternity.