The ZX81 is a home computer developed by Sinclair Research and manufactured in Dundee, Scotland, by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair's ZX80 and designed to be a low-cost introduction to home computing for the general public. It was hugely successful; more than 1.5 million units were sold. In the United States it was initially sold as the ZX-81 under licence by Timex. Timex later produced its own versions of the ZX81: the Timex Sinclair 1000 and Timex Sinclair 1500. Unauthorized ZX81 clones were produced in several countries.

The ZX81 was designed to be small, simple, and above all, inexpensive. Video output is for a television set rather than a dedicated monitor. It contains only four silicon chips and 1 KB of RAM. It has no power switch or moving parts, excepting a VHF TV channel selector switch in some models. It has a pressure-sensitive membrane keyboard. Programs and data are loaded and saved onto compact audio cassettes. The ZX81's limitations prompted a market in third-party peripherals to improve its capabilities. Its distinctive case and keyboard brought designer Rick Dickinson a Design Council award.

The ZX81 could be bought by mail order preassembled or, for a lower price, in kit form. It was the first inexpensive mass-market home computer to be sold by high street stores, led by W. H. Smith and soon many other retailers. The ZX81 marked the point when computing in Britain became an activity for the general public rather than businessmen and electronics hobbyists. It produced a community of enthusiasts, some of whom founded businesses developing software and hardware for the ZX81. Many went on to have roles in the British computer industry. The ZX81's commercial success made Sinclair Research one of Britain's leading computer manufacturers and earned a fortune and an eventual knighthood for the company's founder, Sir Clive Sinclair. The system was discontinued in 1984.

The ZX81 has a base configuration of 1 KB of on-board RAM that can officially be expanded externally to 16 KB. Its single circuit board is housed inside a wedge-shaped plastic case measuring 167 millimetres (6.6 in) wide by 40 millimetres (1.6 in) high. The memory is provided by either a single 4118 (1024 bit × 8) or two 2114 (1024 bit × 4) RAM chips. There are only three other onboard chips: a 3.5 MHz Z80A 8-bit microprocessor from NEC, an uncommitted logic array (ULA) chip from Ferranti, and an 8 KB ROM providing a simple Sinclair BASIC interpreter. The entire machine weighs just 350 grams (12 oz). Early versions of the external RAM cartridge contain 15 KB of memory using an assortment of memory chips, while later versions contain 16 KB chips with the lowest addressed kilobyte disabled.

The front part of the case is an integrated 40-key membrane keyboard. It is mechanically simple, consisting of 40 pressure-pad switches and 8 diodes under a plastic overlay, connected in a matrix of 8 rows and 5 columns.

The ZX81 uses a QWERTY keyboard layout displaying 20 graphic and 54 inverse video characters. However ZX81 BASIC commands are not typed in letter by letter, instead each key has up to five key functions. This is how the user displays the ZX81's BASIC keywords, functions, mathematical operations, and graphics.

The ZX81 key's function is determined by a combination of context in the command and mode selection e.g. SHIFT and FUNCTION keys to select the under key keyboard functions. For example, the P key combines the letter P, the " character, and the BASIC commands PRINT and TAB.

Context mode feedback is displayed by the cursor displaying an inverted letter;