Seiko Holdings Corporation (セイコーホールディングス株式会社 Seikō Hōrudingusu Kabushiki-gaisha?) (TYO: 8050), more commonly known simply as Seiko (/ˈseɪko�/ say-koh), is a Japanese company that manufactures and sells watches, clocks, electronic devices, semiconductors and optical products.

The company was founded in 1881, when Kintarō Hattori opened a watch and jewelry shop called "K. Hattori" (服部時計店 Hattori Tokeiten?) in the Ginza area of Tokyo, Japan. Eleven years later, in 1892, he began to produce clocks under the name Seikosha (精工舎 Seikōsha?), meaning roughly "House of Exquisite Workmanship". According to Seiko's official company history, titled "A Journey In Time: The Remarkable Story of Seiko" (2003), Seiko is a Japanese word meaning "exquisite" or "success" ("exquisite" is usually written 精巧 from Chinese jīngqiǎo, while the meaning "success" is usually written 成功 from Chinese chénggōng)

The first watches produced under the Seiko brand appeared in 1924. In 1969, Seiko introduced the Astron, the world's first production quartz watch; when it was introduced, it cost the same as a medium-sized car. Seiko later went on to introduce the first quartz chronograph. In 1985, Orient and Seiko established a joint factory.

The company was incorporated (K. Hattori & Co., Ltd.) in 1917 and was renamed Hattori Seiko Co., Ltd. in 1983 and Seiko Corporation in 1990. After reconstructing and creating its operating subsidiaries (such as Seiko Watch Corporation and Seiko Clock Inc.), it became a holding company in 2001 and was renamed Seiko Holdings Corporation as of July 1, 2007.

Seiko is perhaps best known for its wristwatches, all of which were at one time produced entirely in-house. This includes not only major items such as microgears, motors, hands, crystal oscillators, batteries, sensors, LCDs but also minor items such as the oils used in lubricating the watches and the luminous compounds used on the hands and the dials. Currently watch movements are made in Shizukuishi, Iwate (SII Morioka Seiko Instruments), Ninohe, Iwate (SII Ninohe Tokei Kogyo), Shiojiri, Nagano (Seiko Epson) and their subsidiaries in China, Malaysia and Singapore. The fully integrated in-house production system is still practised for luxury watches in Japan.

Seiko produces both quartz and mechanical watches of varying prices. The least expensive are around ¥4,000 (US$45) (sold under the brand Alba); the most expensive (Credor JURI GBBX998) costs ¥50,000,000  (US$554,000)

Seiko mechanical watches are highly prized by collectors: from the Seiko "5" series (the 5 reflects five key features of the watch, namely automatic winding, day and date display in a single window—rare at the time, water resistant, recessed crown at the 4 o’clock position and durable case and bracelet—i.e. steel),[5] which is the most common, to the highly prized luxury "Credor," "King Seiko," and "Grand Seiko" lines.