Mitsui Group is a Japanese corporate group and keiretsu that traces its roots to the zaibatsu groups that were dissolved after World War II. Unlike the zaibatsu of the pre-war period, there is no controlling company with regulatory power. Instead, the companies in the group hold shares in each other, but they are limited to exchanging information and coordinating plans through regular meetings.

A keiretsu 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 is loosely in an organized alliance 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. 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.