The ZX Spectrum (UK: /zɛd ɛks/) is an 8-bit home computer developed and marketed by Sinclair Research. The Spectrum played a pivotal role in the history of personal computers and video games, especially in the United Kingdom. It was one of the all-time bestselling British computers with over five million units sold. It was released in the UK on 23 April 1982, the United States in 1983, and Europe in 1984.

The machine was designed by the English entrepreneur and inventor Sir Clive Sinclair and his small team in Cambridge, and was manufactured in Dundee, Scotland by Timex Corporation. It was made to be small, simple, and most importantly inexpensive, with as few components as possible. The addendum "Spectrum" was chosen to highlight the machine's colour display, which differed from the black-and-white display of its predecessor, the ZX81. Rick Dickinson designed its distinctive case, rainbow motif, and rubber keyboard. Video output is transmitted to a television set rather than a dedicated monitor, while application software is loaded and saved onto compact audio cassettes.

The ZX Spectrum was initially distributed by mail order, but after severe backlogs it was sold through High Street chains in the United Kingdom. It was released in the US as the Timex Sinclair 2068 in 1983, and in some parts of Europe as the Timex Computer 2048. There are seven models overall, ranging from the entry level with 16 KB RAM released in 1982 to the ZX Spectrum +3 with 128 KB RAM and built-in floppy disk drive in 1987. The machine primarily competed with the Commodore 64, BBC Micro, Dragon 32, and the Amstrad CPC range. Over 24,000 software products were released for the ZX Spectrum.

Its introduction led to a boom in companies producing software and hardware, the effects of which are still seen. It was among the first home computers aimed at a mainstream UK audience, with some crediting it for launching the British information technology industry. The Spectrum was Britain's top-selling computer until the Amstrad PCW surpassed it in the 1990s. It was discontinued in 1992.

The ZX Spectrum was conceived and designed by engineers at Sinclair Research, founded by English entrepreneur and inventor Clive Sinclair, who was well known for his eccentricity and pioneering ethic. On 25 July 1961, three years after passing his A-levels, he founded Sinclair Radionics to advertise his inventions and buy components. In 1972, Sinclair competed with Texas Instruments to produce the world's first pocket calculator, the Sinclair Executive. By the mid 1970s, Sinclair Radionics was producing handheld electronic calculators, miniature televisions, and the ill-fated digital Black Watch wristwatch. Due to financial losses, Sinclair sought investors from the National Enterprise Board (NEB), who had bought a 43% interest in the company and streamlined his product line. Sinclair's relationship with the NEB had worsened, and by 1979 it opted to break up Sinclair Radionics entirely, selling off its television division to Binatone and its calculator division to ESL Bristol.

After incurring a £7 million investment loss, Sinclair was given a golden handshake and an estimated £10,000 severance package. He had a former employee, Christopher Curry, establish a "corporate lifeboat" company named Science of Cambridge Ltd, in July 1977, called such as they were located near the University of Cambridge. By this time inexpensive microprocessors had started appearing on the market, which prompted Sinclair to start producing the MK14, a computer teaching kit which sold well at a very low price. Encouraged by this success, Sinclair renamed his company to Sinclair Research, and started looking to manufacture personal computers. Keeping the cost low was essential for Sinclair to avoid his products from becoming outpriced by American or Japanese equivalents as had happened to several of the previous Sinclair Radionics products. On 29 January 1980, the ZX80 home computer was launched to immediate popularity; notable for being one of the first computers available in the United Kingdom for less than £100. The company conducted no market research whatsoever prior to the launch of the ZX80; according to Sinclair, he "simply had a hunch" that the public was sufficiently interested to make such a project feasible and went ahead with ordering 100,000 sets of parts so that he could launch at high volume.

On 5 March 1981, the ZX81 was launched worldwide to immense success with more than 1.5 million units sold, 60% of which was outside Britain. According to Ben Rosen, by pricing the ZX81 so low, the company had "opened up a completely new market among people who had never previously considered owning a computer". After its release, computing in Britain became an activity for the general public rather than the preserve of office workers and hobbyists. The ZX81's commercial success made Sinclair Research one of Britain's leading computer manufacturers, with Sinclair himself reportedly "amused and gratified" by the attention the machine received.

Development of the ZX Spectrum began in September 1981, a few months after the release of the ZX81. Sinclair resolved to make his own products obsolete before his rivals developed the products that would do so. Parts of designs from the ZX80 and ZX81 were reused to ensure a speedy and cost-effective manufacturing process. The team consisted of 20 engineers housed in a small office at 6 King's Parade, Cambridge. During early production, the machine was known as the ZX81 Colour or the ZX82 to highlight the machine's colour display, which differed from the black and white of its predecessors. The addendum "Spectrum" was added later on, to emphasise its 15-colour palette. Aside from a new crystal oscillator and extra chips to add additional kilobytes of memory, the ZX Spectrum was intended to be, as quoted by Sinclair's marketing manager, essentially a "ZX81 with colour". According to Sinclair, the team also wanted to combine the ZX81's separate random-access memory sections for audio and video into a single bank.