Lear Siegler Incorporated (LSI) is a diverse American corporation established in 1962. Its products range from car seats and brakes to weapons control systems for military fighter planes. The company's more than $2 billion-a-year annual sales comes from three major areas: aerospace-technology, automotive parts, and industrial-commercial. The company, however, is basically anonymous, since its products are either unmarked or bear only the label "LSI". Lear Siegler went private in 1987.

LSI is sometimes confused with Learjet, which manufactures executive jets.

The Siegler Corporation was incorporated in December 1950 as the Siegler Heating Company. Originally a maker of climate control equipment, the company changed its name to Siegler Corporation after merging with Siegler Enamel Range Company Inc. in 1954. In that year, John G. Brooks, a flamboyant entrepreneur, and nine other associates bought the Siegler Corporation of Centralia, Illinois, for $3.3 million; $3.2 million of this was borrowed for 24 hours at a cost of $60,000. Over the next decade and a half, Brooks, who became Siegler's first president, established a reputation for supervising numerous startling acquisitions. In June 1955, seven months after the merger, Hallamore Manufacturing Company, an electronics firm, became Siegler's first "technology" acquisition. During the 1960s, the company expanded rapidly. John G. Brooks (Echo Products, Zenith Radio, and US Army Air Corps) headed up the new enterprise.

The management team transformed the low-tech space heater company into a viable corporate platform for acquiring multiple successful small companies. This strategy of "buying growth" coupled with sound management proved successful. At the end of its first year the corporation listed on the New York Stock Exchange. The Siegler Corporation quickly distinguished itself as one of the first conglomerates. These were a new breed of business entities that were characterized by a variety of diverse business interests or operating divisions held or controlled by a central management. This management component was typically the only thing these divisions had in common. Examples of conglomerates are LTV (Ling-Temco-Vought) and TRW (Thompson Ramo Wooldridge).

Siegler continued its program of non-hostile acquisitions of target companies. In 1956–1957 it added Hallamore Electronics of Anaheim, California, and the corporate offices followed to Southern California. Notable efforts of the new Los Angeles based enterprise included:

Lear Siegler Incorporated was created as a result of the 1961 merger between the Siegler Corporation (Los Angeles) and Lear Avionics Inc. (of Santa Monica, also known as Lear Inc.). Lear Inc. was an aerospace electronics firm.

The merger was complete by 1962, and the new company was named Lear Siegler Incorporated. The deal, which cost Siegler five shares for each seven Lear shares, nearly doubled the company's sales – from $96.2 million in 1961 to $190.8 million by the end of 1962.

John G. Brooks was founder, President and Chairman of Siegler; and William Lear was founder, President and chairman at Lear.