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 (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:

• in 1984, the Life Science Center in Tsukuba
• in 1995, the Muon Research Facility at the Rutherford Appleton Laboratory in the UK
• in 1997, the Harima Institute, the Brain Science Institute in Wako, and the center at the Brookhaven National Laboratory in the USA
• in 1998, the Genomic Sciences Center
• in 2000, the Yokohama Institute, which now contains four centers for research in the life sciences
• in 2002, the Kobe Institute, which contains the Center for Developmental Biology

In October 2003, Riken's status changed again, to Independent Administrative Institution. As such, Riken is still publicly funded, and it is periodically evaluated by the government, but it has a higher degree of autonomy than before. Riken is regarded as the flagship research institute in Japan and conducts basic and applied experimental research in a wide range of science and technology fields including physics, chemistry, medical science, biology and engineering.

Riken was the subject of international attention in 2014 after the Stimulus-triggered acquisition of pluripotency cell (also known as STAP) publication, investigation, retraction, and suicide of Yoshiki Sasai, the principal investigator.

The main divisions of Riken are listed here. Purely administrative divisions are omitted.

• Headquarters (mostly in Wako)
• Wako Branch
  • Center for Emergent Matter Science (research on new materials for reduced power consumption)
  • Center for Sustainable Resource Science (research toward a sustainable society)
  • Nishina Center for Accelerator-Based Science (site of the Radioactive Isotope Beam Factory, a heavy-ion accelerator complex)
  • Center for Brain Science
  • Center for Advanced Photonics (research on photonics including terahertz radiation)
  • Research Cluster for Innovation
  • Cluster for Pioneering Research (chief scientists)
  • Interdisciplinary Theoretical and Mathematical Sciences Program
• Tokyo Branch
  • Center for Advanced Intelligence Project (research on artificial intelligence)
• Tsukuba Branch
  • BioResource Research Center
• Harima Institute
  • Riken SPring-8 Center (site of the SPring-8 synchrotron and the SACLA x-ray free electron laser)
• Yokohama Branch (site of the Yokohama Nuclear magnetic resonance facility)
  • Center for Sustainable Resource Science
  • Center for Integrative Medical Sciences (research toward personalized medicine)
  • Center for Biosystems Dynamics Research (also based in Kobe and Osaka)
  • Program for Drug Discovery and Medical Technology Platform
  • Structural Biology Laboratory
  • Sugiyama Laboratory
• Kobe Branch
  • Center for Biosystems Dynamics Research (developmental biology and nuclear medicine medical imaging techniques)
  • Center for Computational Science (R-CCS, home of the Fugaku super computer and now decommissioned K computer)

• Two Riken scientists have won the Nobel prize for physics: Hideki Yukawa in 1949 and Shin'ichirō Tomonaga in 1965.
• The SPring-8 (Super Photon Ring 8GeV) facility in Harima is one of the largest and most powerful third-generation synchrotron radiation facility.
• In July 2004 a team at Riken created element 113 (now named nihonium, after Nihon = Japan). On April 2, 2005 the same team successfully created it for the second time, and a third event was seen in 2012. The discovery was officially recognized by the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) in December 2015.
• The Riken Super Combined Cluster is one of the world's fastest supercomputers. In January 2006, Riken set up the Next-Generation Supercomputer R&D Center, with the purpose of designing and building the fastest supercomputer in the world, and in June 2006, it announced the completion of a one-petaFLOPS computer system designed specially for molecular dynamics simulation. Currently a new system, the K computer is being installed at Riken and despite it being still not finished, it topped the LINPACK benchmark with the performance of 8.162 petaFLOPS, or 8.162 quadrillion calculations per second, with a computing efficiency ratio of 93.0%, making it the fastest supercomputer in the world at the time. The complete project entered service in November 2012.

• Dairoku Kikuchi (1917)
• Kōi Furuichi (1917–1921)
• Masatoshi Ōkōchi (1921–1946)
• Yoshio Nishina (1946–1951)
• Kiichi Sakatani (1951–1952)
• Takeshi Murayama (1952–1956)
• Masanori Satō (1956–1958)
• Haruo Nagaoka (1958–1966)
• Shirō Akahori (1966–1970)
• Toshio Hoshino (1970–1975)
• Shinji Fukui (1975–1980)
• Tatsuoki Miyajima (1980–1988)
• Minoru Oda (1988–1993)
• Akito Arima (1993–1998)
• Shunichi Kobayashi (1998–2003)
• Ryōji Noyori (2003 – 31 March 2015)
• Hiroshi Matsumoto (1 April 2015 – 31 March 2022)
• Makoto Gonokami (1 April 2022–present)

• Riken integrated database of mammals

• Official website (in English and Japanese)
• Riken Research (in English and Japanese) – A resource for up-to-date information on key achievements of Riken researchers.