In common law, posse comitatus (Latin for "the power of the county") referred to the authority wielded by the county sheriff to conscript any able-bodied male over the age of fifteen to assist him in keeping the peace or to pursue and arrest a felon; compare hue and cry. It is the law enforcement equivalent of summoning the militia for military purposes.

With modern methods of law enforcement, the posse comitatus is generally obsolete. The power presumably continues to exist in those U.S. states that have not repealed it by statute, however. Resort to the posse comitatus figures often in the plots of Western movies, where the body of men recruited is frequently referred to as a posse. Based on this usage, the word posse has come to be used colloquially to refer to various teams, cliques, or gangs. In a number of states, especially in the western United States, sheriffs and other law enforcement agencies have called their civilian auxiliary groups "posses."

This use of the word became less frequent after a TV talk show scandal about a group of young men in Los Angeles, California who called themselves the "Spurs Posse," apparently after the San Antonio Spurs. They achieved notoriety through a competition involving scoring points for sexual intercourse, and became the subject of a minor moral panic in 1994.

In the United States, a Federal statute known as the Posse Comitatus Act forbids the use of the military of the United States from being used as a posse comitatus or for law enforcement purposes.

The practical disuse of the posse comitatus, and its continued twilight existence as a theoretical legal power, is like the militia a subject for the debates about the meaning of the U.S. Constitution Second Amendment.

Fradulent use

Because of the relationship of a posse comitatus to an armed citizenry, in the U.S. a loose-knit anti-Semitic terrorist organization or group of organizations calls itself The Posse Comitatus. They believe that Jews dominate the Federal government and control its fiat money, and refuse payment of taxes and debts for that reason; they file worthless legal documents proclaiming independence from the United States, or claiming liens against Internal Revenue Service employees, judges, and other perceived enemies. They claim that the county sheriff is the supreme executive authority in the United States. The group has been involved in murder of United States Marshals and other serious crimes.