Julius Kahn (March 8, 1874 – November 4, 1942) was an American engineer, industrialist, and manufacturer. He was the inventor of the Kahn system, a reinforced concrete engineering technique for building construction. The Kahn system, which he patented in 1903, was used worldwide for housing, factories, offices and industrial buildings. He formed his own company, Trussed Concrete Steel Company, as a manufacturing source for his inventions. He also founded United Steel Company and was chairman of Truscon Laboratories.

Kahn was born in Münstereifel, Germany, March 8, 1874. He immigrated to the United States with his family in 1880, entering at the port of Baltimore, Maryland, where they briefly lived. They continued to Detroit, Michigan, where he was raised, along with five brothers and two sisters.

Their father Joseph (1860–1924) was a rabbi and ran a restaurant where Julius worked; he also sold newspapers. Mother Rosalie was skilled in music and art. Kahn's elementary education was in Detroit Public Schools. He completed the normal four-year high school curriculum in three years.

Kahn, assisted financially by his older brother, architect Albert, attended the University of Michigan. He received a Bachelor of Science and a degree in Civil Engineering in 1896.

After graduating from the University of Michigan, Kahn began his career as an engineering draftsman for the Union Bridge Company of New York. He worked as an engineer for both the United States Navy and the U.S. Army Corps of Engineers from 1896 to 1903. Kahn also was employed by C. W. Hunt Company of New York. In 1900, Kahn moved to Japan for two years, laboring in engineering, construction, and maintenance of iron and sulfur mines.

Kahn returned to Detroit in 1903, joining Albert Kahn Associates, an architectural firm founded by his brother Albert in 1895. Kahn's first assignment was collaborating with key architect Ernest Wilby in Ann Arbor on the University of Michigan's new College of Engineering building. Kahn's focus was testing specific materials for sufficient strength in supporting the new building.

Kahn and Wilby wanted to use reinforced concrete for the building's floors, in place of traditional wood supports. Kahn had previous experience in reinforced concrete with the U.S. Army Corps of Engineers. During the construction of U.S. War College building in Washington, D.C., he initiated methods improving the then existing technology of reinforced concrete by beginning work developing the Kahn system of steel bars. This new concept for reinforced concrete steel bars was the basis of the steel production company he later founded.

Kahn understood the structural challenges inherent in the existing method of concrete reinforcement used at the end of the nineteenth century. The main problem was slippage of the steel within the hardened concrete, which weakened the structure. He experimented in his brother's basement, where he developed an improved type of reinforced beam called "the Kahn Bar". Kahn patented his invention in 1903, the first of more than 75 patents awarded to him.