A keiretsu (???, lit. system or series) is a set of companies with interlocking business relationships and shareholdings. It is a type of business group. The prototypical keiretsu are those which appeared in Japan during the "economic miracle" following World War II. Before Japan's surrender, Japanese industry was controlled by large family-controlled vertical monopolies called zaibatsu. The Allies dismantled the zaibatsu in the late 1940s, but the companies formed from the dismantling of the zaibatsu were reintegrated. The dispersed corporations were re-interlinked through share purchases to form horizontally-integrated alliances across many industries. Where possible, keiretsu companies would also supply one another, making the alliances vertically integrated as well. In this period, official government policy promoted the creation of robust trade corporations which could withstand heavy pressures from intensified world trade competition

The major keiretsu were each centered around one bank, which lent money to the keiretsu's member companies and held equity positions in the companies. Each central bank had great control over the companies in the keiretsu and acted as a monitoring entity and as an emergency bail-out entity. One effect of this structure was to minimize the presence of hostile takeovers in Japan, because no entities could challenge the power of the banks.

There are two types of keiretsu: vertical and horizontal. Vertical keiretsu illustrates the organization and relationships within a company (for example all factors of production of a certain product will be connected), while a horizontal keiretsu shows relationships between entities and industries, normally centered around a bank and trading company. Both are complexly woven together and self-sustain each other.

The Japanese recession in the 1990s had profound effects on the keiretsu. Many of the largest banks were hit hard by bad loan portfolios and forced to merge or go out of business. This had the effect of blurring the lines between the keiretsu: Sumitomo Bank and Mitsui Bank, for instance, became Sumitomo Mitsui Banking Corporation in 2001, while Sanwa Bank (the banker for the Hankyu-Toho Group) became part of Bank of Tokyo-Mitsubishi UFJ. Additionally, many companies from outside the keiretsu system, such as Sony, began outperforming their counterparts within the system.

Generally, these causes gave rise to a strong notion in the business community that the old keiretsu system was not an effective business model, and led to an overall loosening of keiretsu alliances. While the keiretsu still exist, they are not as centralized or integrated as they were before the 1990s. This, in turn, has led to a growing corporate acquisition industry in Japan, as companies are no longer able to be easily "bailed out" by their banks, as well as rising derivative litigation by more independent shareholders.

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