Ichitaro (一太郎, ichitarō) is a Japanese word processor produced by JustSystems, a Japanese software company. Ichitaro occupies the second share in Japanese word-processing software, behind Microsoft Word. It is one of the main products of the company. Its proprietary file extension is ".JTD". ATOK, an IME developed by JustSystems, is bundled with Ichitaro.

In the MS-DOS era, Ichitaro had a considerable market share along with other rivals. As Windows became dominant, the market was largely taken over by Microsoft Word.

"Taro (太郎)" was named by Kazunori Ukigawa (浮川 和宣), a founder of JustSystems. When he worked part-time as a tutor, one of his learners' names was Taro. He died of sickness when Kazunori worked at the company. Taro is also a common Japanese given name used for the eldest son. Sanyo Electric already had a trademark right of the name, so JustSystems added a prefix "Ichi (一)" as they hoped the software won the best.

JustSystems was founded in July 1979 by Hatsuko (初子) and Kazunori Ukigawa, and was incorporated in June 1981. Kazunori worked at a subsidiary company of Toshiba, and he was interested in Japanese-language computing. Toshiba released JW-10 [ja] in February 1979, the first word processor for the Japanese language, but it sold less than their business computers. He founded the company as a dealer of business computers, and they started selling Japanese language software. After the release of PC-8801, they developed an invoicing software for it, which printed out estimations and invoices in Japanese. They demonstrated it at trade fairs, and received a positive response.

When they contacted to ASCII Microsoft about a run-time license fee of the BASIC compiler used for their farm management software, ASCII had known JustSystem's Japanese software, so they asked JustSystems to develop a Japanese word processor software for PC-100. It was released as JS-WORD in 1983. It was followed by JS-WORD 2.0 ported for PC-9801. Both were published by ASCII under their name. JS-WORD featured mouse support and a graphical icon-based interface, but it resulted in poor performance. The jX-WORD for the IBM JX was released and in 1985, jX-WORD Taro was released for PC-9801. jX-WORD Taro was priced at ¥58,000, which was the middle price among Japanese word processor software, and sold 9,700 copies.

The same year, Ichitaro was released as its definite successor. Ichitaro's system disk contained ATOK 4 and a runtime version of MS-DOS 2.11. It allowed users to use other MS-DOS applications with Japanese language support. The biggest competitor, Matsu (松), was released in 1983 by Kanri Kōgaku Kenkyūjo (管理工学研究所). It gained speed from being written in assembly language and natively ran on PC-9801, but Matsu's Japanese input method couldn't be used for other applications. Ichitaro had plenty of typesettings options, but Matsu did not. Also, Matsu was priced at ¥128,000 before Ichitaro came out.

The first version of Ichitaro has shipped 29,000 copies. Ichitaro Ver.2 has shipped 80,000 copies. Ichitaro Ver.3 was the first version ported for other Japanese DOS platforms. It has shipped more than 300,000 copies until 1991. Total shipments of Ichitaro reached one million in November 1991. Contrary to the Japanese personal computer market expanding into beginners, Matsu oriented for power users, so Ichitaro overtook it.

Ichitaro Ver.4 was released in April 1989. It had a proprietary operating environment called Just Window (ジャストウィンドウ), like Lotus Symphony. Three minor versions were released because it was buggy. The stable version was 4.3 released in December 1989. ATOK 7 was bundled with Ichitaro 4, and was available as a standalone product in 1992.