A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations.

Initially, from 1887 at the earliest, teleprinters were used in telegraphy. Electrical telegraphy had been developed decades earlier in the late 1830s and 1840s, then using simpler Morse key equipment and telegraph operators. The introduction of teleprinters automated much of this work and eventually largely replaced skilled operators versed in Morse code with typists and machines communicating faster via Baudot code.

With the development of early computers in the 1950s, teleprinters were adapted to allow typed data to be sent to a computer, and responses printed. Some teleprinter models could also be used to create punched tape for data storage (either from typed input or from data received from a remote source) and to read back such tape for local printing or transmission. A teleprinter attached to a modem could also communicate through telephone lines. This latter configuration was often used to connect teleprinters to remote computers, particularly in time-sharing environments.

Teleprinters have largely been replaced by fully electronic computer terminals which typically have a computer monitor instead of a printer (though the term "TTY" is still occasionally used to refer to them, such as in Unix systems). Teleprinters are still widely used in the aviation industry (see AFTN and airline teletype system), and variants called Telecommunications Devices for the Deaf (TDDs) are used by the hearing impaired for typed communications over ordinary telephone lines.

The teleprinter evolved through a series of inventions by a number of engineers, including Samuel Morse, Alexander Bain, Royal Earl House, David Edward Hughes, Emile Baudot, Donald Murray, Charles L. Krum, Edward Kleinschmidt and Frederick G. Creed. Teleprinters were invented in order to send and receive messages without the need for operators trained in the use of Morse code. A system of two teleprinters, with one operator trained to use a keyboard, replaced two trained Morse code operators. The teleprinter system improved message speed and delivery time, making it possible for messages to be flashed across a country with little manual intervention.

There were a number of parallel developments on both sides of the Atlantic Ocean. In 1835 Samuel Morse devised a recording telegraph, and Morse code was born. Morse's instrument used a current to displace the armature of an electromagnet, which moved a marker, therefore recording the breaks in the current. Cooke & Wheatstone received a British patent covering telegraphy in 1837 and a second one in 1840 which described a type-printing telegraph with steel type fixed at the tips of petals of a rotating brass daisy-wheel, struck by an "electric hammer" to print Roman letters through carbon paper onto a moving paper tape. In 1841 Alexander Bain devised an electromagnetic printing telegraph machine. It used pulses of electricity created by rotating a dial over contact points to release and stop a type-wheel turned by weight-driven clockwork; a second clockwork mechanism rotated a drum covered with a sheet of paper and moved it slowly upwards so that the type-wheel printed its signals in a spiral. The critical issue was to have the sending and receiving elements working synchronously. Bain attempted to achieve this using centrifugal governors to closely regulate the speed of the clockwork. It was patented, along with other devices, on April 21, 1841.

By 1846, the Morse telegraph service was operational between Washington, D.C., and New York. Royal Earl House patented his printing telegraph that same year. He linked two 28-key piano-style keyboards by wire. Each piano key represented a letter of the alphabet and when pressed caused the corresponding letter to print at the receiving end. A "shift" key gave each main key two optional values. A 56-character typewheel at the sending end was synchronised to coincide with a similar wheel at the receiving end. If the key corresponding to a particular character was pressed at the home station, it actuated the typewheel at the distant station just as the same character moved into the printing position, in a way similar to the (much later) daisy wheel printer. It was thus an example of a synchronous data transmission system. House's equipment could transmit around 40 instantly readable words per minute, but was difficult to manufacture in bulk. The printer could copy and print out up to 2,000 words per hour. This invention was first put in operation and exhibited at the Mechanics Institute in New York in 1844.

Landline teleprinter operations began in 1849, when a circuit was put in service between Philadelphia and New York City.