Ralph Modjeski (born Jan. 27, 1861,Kraków, Pol.—died June 26, 1940, Los Angeles) was a Polish-born American bridge designer and builder, outstanding for the number, variety, and innovative character of his projects.

He was the son of the actressHelena Modjeska (1840–1909). After study in Paris, he settled in theUnited States and from 1892 practiced as a consulting bridge engineer inChicago.

Among his best known bridges were the seven-span railroad and highway bridge over the Mississippi atRock Island, Ill.; the McKinley Bridge at St. Louis, Mo.; the Northern Pacific railroad bridge over the Missouri at Bismarck, N.D.; and bridges over the Columbia and Willamette rivers in Oregon. His double-track railroad bridge over theOhio River at Metropolis, Ill., contained several strikinginnovations, including a simple truss span of 720 feet (219 metres) and the unusual use of alloys: silicon steel for the bridge proper and nickel steel for the tension members. He was chief engineer and chairman of the board of engineers of theBenjamin Franklin Bridge over theDelaware River, which, upon completion in 1926, was the longestsuspension bridge in the world.

Modjeski was also chief engineer of theHuey P. Long Bridge over the Mississippi atNew Orleans and, as his last undertaking, served as chairman of the board of consulting engineers for the San Francisco–OaklandBay Bridge (California), completed in 1936. By the time he died, he had been associated with more than 50 major bridges.