Riken (Japanese: 理研; English: /ˈrɪkɛn/; stylized in all caps as RIKEN) is a national scientific research institute in Japan. Founded in 1917, it now has about 3,000 scientists on seven campuses across Japan, including the main site at Wakō, Saitama Prefecture, on the outskirts of Tokyo. Riken is a Designated National Research and Development Institute, and was formerly an Independent Administrative Institution.

Riken conducts research in various fields of science, including physics, chemistry, biology, genomics, medical science, engineering, high-performance computing and computational science, and ranging from basic research to practical applications with 485 partners worldwide. It is almost entirely funded by the Japanese government, with an annual budget of ¥100 billion (US$750 million) in FY2023.

"Riken" is an acronym of the formal name Rikagaku Kenkyūsho (理化学研究所), and its full name in Japanese is Kokuritsu Kenkyū Kaihatsu Hōjin Rikagaku Kenkyūsho (国立研究開発法人理化学研究所) and in English is the Institute of Physical and Chemical Research.

In 1913, the well-known scientist Jokichi Takamine first proposed the establishment of a national science research institute in Japan. This task was taken on by Viscount Shibusawa Eiichi, a prominent businessman, and following a resolution by the Diet in 1915, Riken came into existence in March 1917. In its first incarnation, Riken was a private foundation (財団法人, zaidan), funded by a combination of industry, the government, and the Imperial Household. It was located in the Komagome district of Tokyo, and its first director was the mathematician Baron Dairoku Kikuchi.

In 1927, Viscount Masatoshi Ōkōchi, the third director, established the Riken Concern (a zaibatsu). This was a group of spin-off companies that used Riken's scientific achievements for commercial ends and returned the profits to Riken. At its peak in 1939 the zaibatsu comprised about 121 factories and 63 companies, including Riken Kankōshi, which is now Ricoh.

During World War II, the Japanese army's atomic bomb program was conducted at Riken. In April 1945 the US bombed Riken's laboratories in Komagome, and in November, after the end of the war, Allied soldiers destroyed its two cyclotrons.

After the war, the Allies dissolved Riken as a private foundation, and it was brought back to life as a company called Kagaku Kenkyūsho (科学研究所), or Kaken (科研). In 1958 the Diet passed the Riken Law, whereby the institute returned to its original name and entered its third incarnation, as a public corporation (特殊法人, tokushu hōjin), funded by the government. In 1963 it relocated to a large site in modern day Wakō then until 1970 in Saitama Prefecture, just outside Tokyo.

Since the 1980s Riken has expanded dramatically. New labs, centers, and institutes have been established in Japan and overseas, including: