Seiko Instruments Inc. (セイコーインスツル株式会社, Seikō Insutsuru Kabushiki-gaisha) (SII) is a Japanese company, which develops and commercializes semiconductor, micromechatronics, and precision machining technologies.

Headquartered in the Makuhari business district, Mihama-ku, Chiba City, Chiba Prefecture, Japan, the company manufactures and sells electronic components (crystal oscillators, micromechatronics devices, piezo inkjet printheads, microbatteries, supercapacitors), precision parts, analysis and measurement instruments, machine tools, factory automation systems, printers, etc.

In 1937, Daini Seikosha Co., Ltd. (第二精工舎, Dai-ni-seikōsha), literally the second workshop for manufacturing Seiko timepieces, was established in Kamedo, Kōtō, Tokyo as a spin-off of the watch manufacturing division from Seikosha (精工舎, Seikōsha), so had been making the Seiko watches until 2020.

Seiko had delegated a large portion of the manufacturing in its watch business to SII.

On April 1, 2020, the company transferred its watch business including its watch manufacturing subsidiaries Morioka Seiko Instruments, Ninohe Tokei Kogyo, Seiko Instruments Singapore, etc.