HKS Co., Ltd. (株式会社エッチ・ケー・エス, Kabushiki-gaisha Ecchi Kē Esu) is a publicly traded company headquartered in Fujinomiya, Shizuoka Prefecture, Japan. The company specializes in the engineering, manufacturing, and sales of high performance aftermarket and accessory automotive parts and components. With 50 years in business, including global efforts in aftermarket parts and motorsport, the company claims, "HKS is perhaps the most known aftermarket brand in the world."

HKS was formed in 1973 by Hiroyuki Hasegawa, a former engineer for the Yamaha Motor Company, and his partner Goichi Kitagawa, while the start up capital was supplied by Sigma Automotive (hence the name HKS). The company began operations by tuning gasoline engines in a dairy-farming shed at the foot of Mount Fuji. Their goal was to design and build high performance engines and components that major original equipment manufacturers could not, or would not, produce.

In July 1974, Hasegawa engineered and built the first commercialized turbocharger kit for passenger automobiles. Since then HKS has been developing turbocharger upgrades and bolt-on turbocharger kits that subsequently became the company's core business. HKS also created the first commercially available electronic turbo timer and boost controller, and also pioneered piggy-back fuel computers and aftermarket fuel management tools.

In 1984, HKS brought in the Nara-based tuning and service shop Twin Power Co., Ltd. under the new name of HKS Kansai Service Co., Ltd. as an affiliated partner with HKS. The shop served as a service center, tuning partner, parts distributor, and R&D partner for HKS. In 2002, HKS Kansai Service became a major suspension overhaul and servicing center for HKS suspension customers. HKS Kansai Service also gained their own accolades and accomplishments in motorsport and even began developing their own aftermarket parts. In 2010, the official affiliation came to a close and the company was renamed to Kansai Service Co., Ltd. but it still maintains a good working relationship with HKS.

In July 1984, HKS built a new factory in Kamiide, Fujinomiya, not far from the site of the original shop at the Mount Fuji dairy-farming shed. In April 1985, HKS built an additional factory in Kitayama specifically for muffler production for both HKS and on consignment. In 2003, the first HKS Technical Factory was established in Toda, Saitama and later another was established in Sapporo. These technical factories serve as HKS-owned service, tuning, and development centers, as well as offering the sale of select HKS complete vehicles and parts. Additionally, a dedicated HKS Service Center was also established in Kitakyushu. The company also operates three sales offices within Japan, in Tokyo, Nagoya, and Osaka. The HKS headquarters remains near Mount Fuji with its main 100,000 square foot manufacturing and R&D facility in Fujinomiya City, Shizuoka Prefecture with an international sales and distribution network spanning Asia, Europe, Australia and the Americas to support its customer base.

The subsidiary HKS USA was originally established in 1982 but shut down operations in 2011, with HKS electing instead to use wholesale distributors to handle their supply chain in the USA. Motovicity Distribution was selected as the North American master distributor in 2011. HKS USA was reestablished in 2017 as the consolidated subsidiary HKS USA, Inc. in Chandler, Arizona. In May 2020, Motovicity Distribution's assets were acquired by Turn 14 Distribution for authorized distribution of HKS's products in the United States. Additional subsidiary companies have been established in Cambridgeshire, England (HKS Europe) in 1996, Bangkok, Thailand (HKS Thailand) in 2001, and Shanghai, China (HKS Shanghai) in 2012.

Since June 1999, HKS has been a publicly traded company, first with the Japan Securities Dealers Association, and later in 2004 on the JASDAQ Securities Exchange.

HKS also gained full ownership of the metal processing and manufacturing company Nissei Kogyo Co., Ltd. in May, 2000. Nissei Industries also produces crankshafts, camshafts, cylinders, pistons, and injector parts.