Onoto is a British brand of luxury fountain pens and accessories. Originally manufactured by Thomas De La Rue & Company, Limited from 1905 until 1958, the brand was relaunched by "The Onoto Pen Company Limited", based in Colney Hall, Norwich in 2005. Onoto pens have been used by numerous notable people from history such as Field Marshal Douglas Haig and Prime Minister Winston Churchill. Other famous names include Florence Nightingale, Edgar Wallace and Natsume Soseki, the foremost Japanese novelist of the Meiji Era.

The origin of the brand name Onoto is unknown. One suggestion is it is derived from Ono Tokusaburo, a Japanese watchmaker. Another theory is that it was a made-up name chosen specifically for its ease of pronunciation in order to boost global sales.

Thomas De La Rue & Company, Limited, which was one of the world's largest printers of postage stamps, banknotes, and playing cards, had manufactured fountain pens for most of the later part of the 19th century. In 1881 it produced the Anti-Stylograph pre-dating Lewis Waterman's first pen by three years. These writing instruments were well received throughout the British Empire and around the globe.

The Onoto pen was invented by George Sweetser, a mechanical engineer, in 1905. Sweetser offered his invention to Evelyn De La Rue –the eldest son of Thomas De La Rue– who immediately accepted. Other versions state that the Onoto pen was invented jointly by Sweester and Evelyn.

In the autumn of 1905, De La Rue launched a self-filling fountain pen that it guaranteed not to leak because it had a cut-off valve. The writing instrument, which was called the "Onoto Patent Self-filling Pen", also had a patented "plunger filler" system that drew ink into the pen using a vacuum created by the down-stroke of a piston. All pens were handmade in Bunhill Row, London. Very quickly Onoto pens became popular in Great Britain and internationally. Recognition as a global brand was helped by extensive advertising and marketing. The company spent £50,000 (£2.5 million in 2016) on sales marketing and a wide variety of campaigns in Britain, USA, India, Italy and France. By 1909 Onoto pen advertisements were using the red pillar box and a young boy called "Peter Pen". As a result, a US subsidiary, "Onoto Pen Co.", was opened in New York in 1909.

The strong branding of the "Onoto THE Pen", which sold for £10, (about £500 in 2016) was marketed as the "All British Pen" by the time of the First World War. The strength of the Onoto name was successfully added to other products such as stylographs (a fountain pen with a narrow steel tube instead of a conventional nib), pencils, ink and diaries - and other items such as writing paper, playing cards and blotting paper.

In 1915 De La Rue launched the Onoto Valveless. The Onoto ink pencil and the Onoto Safety 'Receder' (with retractable nib) followed in 1921.

In 1921 the De La Rue family sold the Thomas De La Rue & Company, Limited to a three-company consortium. Two new Onoto models followed, the Onoto metal-cased pencils in 1922 and the first Onoto lever-filler in 1923. Three year later all pen manufacture was transferred to Fife, Scotland in 1927. Production would continue at a disused paper mill at Strathendry which had been operated by J A Weir, one of the companies involved in the consortium.