loose consortium of independent Japanese companies that were created out of the giant, family-owned Mitsui business combine, or zaibatsu, which was broken up after World War II. That zaibatsu, in turn, grew out of the House of Mitsui (Mitsui-ke), the largest Japanese merchant house of the Tokugawa period (1603–1867).

The original enterprise dates from 1673, when Mitsui Takatoshi (1622–94), the son of a sake brewer, opened textile shops in Kyōto and Edo (modern Tokyo). The success of these and subsequent shops allowed him to expand into moneylending and other financial services. Starting in 1691, members of the Mitsui family were designated chartered merchants (goyō shōnin) by the shogunate. These appointments were lucrative for Mitsui and gave it considerable influence in the government. The cultivation of close ties with the government became a great asset to Mitsui, especially during the Meiji period (1868–1912), when the government was encouraging rapid economic development.

In the late 19th century the interests of the emerging Mitsui combine were centred on banking, trading, and mining, activities that Mitsui came to dominate. The family council that had been established in the early 18th century to oversee operations soon became inadequate, and in 1909 it was replaced by a family-controlled holding company. The Mitsui combine underwent tremendous expansion and diversification in the 20th century, becoming the largest zaibatsu in Japan; by the end of World War II it comprised some 270 companies. After Japan's defeat, the Mitsui zaibatsu was split up by U.S. occupation authorities. The holding company was dissolved, and the stock of the former subsidiary companies was sold to the public.

The independent Mitsui companies began to reassociate in the 1950s. In contrast to the structuring of the former zaibatsu, in which the family-dominated holding company had complete control of the combine, the new grouping was characterized by informal policy coordination among the various company presidents and by a degree of financial interdependency among the corporations.

The Mitsui group comprises several dozen companies, including cement, petrochemicals, commerce, construction, energy, engineering, finance and insurance, food, machinery, mining, nonferrous metals, real estate, and shipping industries. All of the major firms are large multinational corporations that are based in Tokyo or Ōsaka and have offices and subsidiaries overseas; some are engaged in joint ventures with foreign companies.

Mitsui and Company, Ltd. (Mitsui Bussan KK), is one of Japan's largest general trading companies and is a major component of the Mitsui group. The company was established as the trading subsidiary of the Mitsui combine in 1876. In the 1950s several of the small trading firms that emerged from the dissolved zaibatsu subsidiary regrouped around the Daiichi Trading Company (Daiichi Bussan Kaisha); the company took its present name in 1959. The firm acts primarily as an agent for other companies, handling orders and arranging shipping for thousands of industrial, agricultural, and consumer goods and commodities. As a leader of the Mitsui group, the company coordinates the domestic and overseas activities of affiliated companies.

Mitsui Taiyo Kobe Bank, Ltd. (Mitsui Taiyo Kobe Ginkō KK), was formed from the merger (1990) of Taiyo Kobe Bank, Ltd., and Mitsui Bank Ltd. (1876), the first private bank in Japan. Mitsui Bank, formed by merger with another bank during World War II, was reestablished as a separate bank in 1948. Taiyo Kobe was formed by merger in 1973. Mitsui Taiyo Kobe is the major financial institution of the Mitsui group, its activities centring on securities and foreign exchange.

Other members of the combine include Mitsui Real Estate Development Company, Ltd., a leading home builder and developer of office and commercial buildings in Japan; Mitsui Mutual Life Insurance Company, one of Japan's major life insurers; Mitsui Mining & Smelting Company, Ltd., engaged in the smelting and processing of zinc, copper, and other nonferrous metals; and Mitsui Mining Company, Ltd., which produces domestic coal, coke, and cement.