SS Illinois was an iron passenger-cargo steamship built by William Cramp & Sons in 1873. The last of a series of four Pennsylvania-class vessels, Illinois and her three sister ships—Pennsylvania, Ohio and Indiana—were the largest iron ships ever built in the United States at the time of their construction, and amongst the first to be fitted with compound steam engines. They were also the first ships to challenge British dominance of the transatlantic trade since the American Civil War.

The Pennsylvania class was a class of four cargo-passenger liners built by the Philadelphian shipbuilder William Cramp & Sons in 1872–73. Intended for the newly established American Line, the four ships—Pennsylvania, Ohio, Indiana and Illinois—were at the time the largest iron ships yet built in the United States, and were launched with considerable fanfare. Upon entering service in 1874, they became the first American-built steamships to challenge British dominance of the transatlantic trade since the American Civil War.