A keiretsu (Japanese: 系列, literally system, series, grouping of enterprises, order of succession) is a set of companies with interlocking business relationships and shareholdings that have dominated the Japanese economy since the second half of the 20th century. In the legal sense, it is a type of informal business group that are loosely organized alliances within the social world of Japan's business community. It rose up to replace the zaibatsu system that was dissolved in the Occupation of Japan following the Second World War, and, though their influence has shrunk since the late 20th century—they continue to be important forces in Japan's economy in the early 21st century.

The members' companies own small portions of the shares in each other's companies, centered on a core bank; this system helps insulate each company from stock market fluctuations and takeover attempts, thus enabling long-term planning in projects. It is a key element of the manufacturing industry in Japan.