The Posse Comitatus (Latin for 'force of the county') is a loosely organized American far-right extremist social movement which began in the late 1960s. Its members spread a conspiracy-minded, anti-government, and antisemitic message linked to white supremacy aiming to counter what they believe is an attack on their social and political rights as white Christians.

Many Posse members practiced survivalism and played a role in the formation of armed citizens' militias in the 1990s. The Posse Comitatus pioneered the use of false liens and other types of "paper terrorism" to harass their opponents by mounting frivolous legal actions against them. As the Posse Comitatus began their decline in popularity at the turn of the 21st century, their tactics and ideology evolved into those of the Christian Patriot movement and the sovereign citizen movement.

Due to the strong ties that they forged with the white supremacist Christian Identity movement, members of the Posse Comitatus believe that they are the true Israelites, the people who were chosen by God. They state that the Jews seek to help Satan destroy civilization and undermine white citizens' rights through the Federal Reserve and the Internal Revenue Service.

Posse charters were first issued in Portland, Oregon, in 1969 by Henry Lamont Beach, "a retired dry cleaner and a one-time member of the Silver Shirts, a Nazi-inspired and clerical fascist organization which was established in America after Hitler took power in Germany." One expert has considered William Potter Gale the founder of the movement.

Posse members believe that there is no legitimate form of government above that of the county level and no higher law authority than the county sheriff. If the sheriff refuses to carry out the will of the county's citizens, "he shall be removed by the Posse to the most populated intersection of streets in the township and at high noon be hung by the neck, the body remaining until sundown as an example to those who would subvert the law."

Some Posse members embraced the antisemitic and white supremacist beliefs of Christian Identity. Some believe that the U.S. federal government is illegitimate and in the hands of a Zionist Occupation Government, the supposed international Jewish conspiracy. Christian Posse Comitatus Newsletter has reported: "Our nation is now completely under the control of the International Invisible government of the World Jewry."

Members of the Posse Comitatus frequently refuse to pay taxes, obtain driver's licenses, or comply with regulatory authorities. They deny the validity of United States fiat money because it is not backed by gold, which they claim the Constitution requires.

They draw up unusual legal documents and attempt to record them, they declare their independence from the United States, or they claim to file "common law" liens against their perceived enemies such as Internal Revenue Service employees or judges. They are frequently involved in various tax protests, and they have invoked ideas that have been popularized by tax protesters.