A Sooner is a term used to describe settlers who entered the Unassigned Lands in what is now the state of Oklahoma before the official start of the Land Rush of 1889. The name derived from the "sooner clause" of Proclamation 288, which stated that anyone who entered and occupied the land prior to the opening time would be denied the right to claim land. The term initially had a negative connotation, as Sooners were essentially stealing from other white settlers by cheating on the claim requirements to get better land. However, these negative connotations rapidly cooled as time passed after 1889 and land claims were settled. Sooners were often deputy marshals, land surveyors, railroad employees, and others who were able to legally enter the territory early. Sooners who crossed into the territory illegally at night were originally called "moonshiners" because they had entered "by the light of the moon". These Sooners would hide in ditches at night and suddenly appear to stake their claim after the land run started, hours ahead of legal settlers.

In 1908, the University of Oklahoma adopted "Sooners" as the nickname of its football team, after having first tried "Rough Riders" and "Boomers". Over time, the term lost its derogatory connotations and began to represent the irrepressible, hardy pioneer spirit of Oklahoma. The word was adopted by the University of Oklahoma in 1908 as their football team’s name, and Oklahoma was unofficially deemed the “Sooner State” in the 20th century. The term now identifies members of University of Oklahoma sports teams, current students and alumni, and devoted fans of the university’s teams.