A turnstile (also called a gateline, baffle gate, automated gate, turn gate in some regions) is a form of gate which allows one person to pass at a time. A turnstile can be configured to enforce one-way human traffic. In addition, a turnstile can restrict passage only to people who insert a coin, ticket, transit pass, security credential, or other method of payment or verification. Modern turnstiles can incorporate biometrics, including retina scanning, fingerprints, and other individual human characteristics which can be scanned.

A turnstile can be used in the case of paid access (sometimes called a faregate or ticket barrier when used for this purpose), for example to access public transport, a pay toilet, or to restrict access to authorized people, for example in the lobby of an office building.

Turnstiles were originally used, like other forms of stile, to allow human beings to pass while excluding livestock, grave yards being one application. They may have been used from the early modern period to control access to toll roads, and retail streets, the Holborn Great Turnstile appearing in 16th century literature Turnstiles were adopted as a means to control access to building and amusements during the 19th century, with many sports stadium and swimming pools installing them, patents for improvements to the concept being granted to manufacturers. Coin and later ticket operated turnstiles appeared on mass transit systems during the 20th century, magnetic strip encoded tickets and passes replacing earlier perforated tickets, during the late 1960s, in Tokyo. The use of turnstiles, in 20th century USA, has been credited to Clarence Saunders, who used them in his first Piggly Wiggly store.

Turnstiles are used at a wide variety of settings, including stadiums, amusement parks, mass transit stations, office lobbies, airports, ski resorts, factories, power plants and casinos.

From a business/revenue standpoint, turnstiles give an accurate, verifiable count of attendance. From a security standpoint, they lead patrons to enter single-file, so security personnel have a clear view of each patron. This enables security to efficiently isolate potential trouble or to confiscate any prohibited materials. On the other hand, physical barriers become a serious safety issue when a speedy evacuation is needed, requiring emergency exits that bypass any turnstiles. The ticket barriers on the London Underground and some busy UK railway stations have to be open if either the gates are unstaffed or in an emergency.

Persons with disabilities may have difficulties using turnstiles. In these cases, generally a wide aisle gate or a manual gate may be provided. At some locations where luggage is expected, a line of turnstiles may be entirely formed of wide aisle gates, for example at Heathrow Terminals 2 & 3 Underground station.

Turnstiles often use ratchet mechanisms to allow the rotation of the stile in one direction allowing ingress but preventing rotation in the other direction. They are often designed to operate only after a payment has been made, usually by inserting a coin or token in a slot; or by swiping, tapping, or inserting a paper ticket or electronically-encoded card.

Turnstiles are often used for counting the numbers of people passing through a gate, even when payment is not involved. They are used extensively in this manner in amusement parks, in order to keep track of how many people enter and exit the park and ride each ride. The, 1873, old Hampden Park stadium in Glasgow, Scotland, was one of the first soccer stadium to install turnstiles.